r 


SRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


;i 


Deceived 
Successions 


MAR '23  1&94 


189 


No 


Class  No. 


ORIGINAL  PAPERS 


ON 


DYNAMO    MACHINERY 


AND  ALLIED  SUBJECTS. 


BY 


JOHN  HOPKINSON,  M.A.,  D.So.,  F.B.S. 


NEW  YORK : 
THE  W.  J.  JOHNSTON  COMPANY,  LIMITBB, 

41  PARK  Row  (TIMES  BUILDING). 


LONDON  : 
WHITTAKER  &  CO., 

2,  WHITEHART  STREET,  PATERNOSTER  SQUARE. 

1893. 


AUTHORIZED  AMERICAN  EDITIOK 


PEEFACE. 


THE  following  short  collection  of  papers  includes  all  that 
I  have  written  of  an  original  character  on  electrotechnical 
subjects.  Here  and  there  errors  have  been  corrected; 
otherwise  the  papers  are  republished  exactly  as  they 
first  appeared.  The  chronological  order  is  not .  strictly 
adhered  to.  The  papers  are  arranged  rather  according  to 
subject.  Thus,  five  papers  relating  wholly  or  in  part  to  the 
continuous  current  dynamo  come  first;  then  follow  four 
on  converters ;  lastly,  there  are  a  note  on  the  theory  of  al- 
ternate current  machines  and  a  paper  on  the  applications 
of  electricity  to  lighthouses. 

The  motive  of  this  publication  has  been  that  I  have 
understood  that  one  or  two  of  these  papers  are  out  of 
print  and  are  not  so  accessible  to  American  readers  as  an 
author  who  very  greatly  values  the  good  opinion  of  Ameri- 
can electrical  engineers  would  desire. 

J.  HOPKI^SOtf. 
LONDON,  September,  1892. 

3 


CONTENTS. 


PAGE 

I.  ON  ELECTRIC  LIGHTING,  .    .    .   - 7 

(Proceedings  of  the  Institution  of  Mechanical  Engineers,  April  25, 1879.) 

II.  ON  ELECTEIC  LIGHTING,  (Second  Paper)    .    .    26 

(Proceedings  of  the  Institution  of  Mechanical  Engineers,  April  23,  1880.) 

III.  SOME  POINTS  IN  ELECTRIC  LIGHTING,  ...    40 

(Proceedings  of  the  Institution  of  Civil  Engineers,  April  5,  1883.) 

IV.  DYNAMO-ELECTRIC  MACHINERY, 79 

(Philosophical  Transactions  of  the  Royal  Society,  May  6,  1886.) 

V.  DYNAMO-ELECTRIC  MACHiNERY,(Second  Paper)  134 

(Proceedings  of  the  Royal  Society,  February  15,  1892.) 

VI.  THEORY  or  ALTERNATING  CURRENTS,  .    .    .  148 

(Institution  of  Electrical  Engineers,  November  13,  1884.) 

VII.  AN  UNNOTICED  DANGER  IN  CERTAIN  APPA- 
RATUS FOR  DISTRIBUTION  OF  ELECTRICITY,  177 

(Philosophical  Magazine,  September,  1885.) 

VIII.  INDUCTION  COILS  OR  TRANSFORMERS,    .    .    .182 

(Proceedings  of  the  Royal  Society,  February  17,  1887.) 

IX.  EEPORT  TO  THE  WESTINGHOUSE  COMPANY  OF 
THE  TEST  OF  Two  6,500-WATT  TRANS- 
FORMERS, MAY  31,  1892, .187 

X.  THEORY  OF  THE  ALTERNATE  CURRENT  DYNAMO,  211 

(Proceedings  of  the  Royal  Society,  February  17,  1887.) 

XL  THE  ELECTRIC  LIGHTHOUSES  OF  MACQUARIE 

AND  OF  TINO, 217 

(Proceedings  of  the  Institution  of  Civil  Engineers,  December  7,  18S6.) 

5 


ORIGINAL  PAPEES 


ON 


DYNAMO    MACHINERY 

AND    ALLIED    SUBJECTS.      • 


ON  ELECTRIC  LIGHTING. 

FIRST   PAPER. 

DURIKG  the  last  year  much  has  been  written  and  much 
information  communicated  concerning  the  production  of 
light  from  mechanical  power  by  means  of  an  electric  cur- 
rent. The  major  portion  of  what  has  appeared  has  been 
either  descriptive  of  particular  machines  for  producing  the 
current,  and  of  lamps  for  manifesting  a  portion  of  its 
energy  as  light,  or  a  statement  of  practical  results  con- 
necting the  light  obtained  with  the  power  applied  and  the 
money  expended  in  producing  it. 

While  fully  appreciating  the  present  value  of  such  in- 
formation, the  author  has  felt  that  it  did  not  tell  all  that 
was  interesting  or  practically  useful  to  know.  It  is  de- 
sirable to  know  what  the  various  machines  can  do  with 
varied  and  known  resistances  in  the  circuit,  and  with  va- 
ried speeds  of  rotation;  and  what  amount  of  power  is 
absorbed  in  each  case.  It  is  a  question  of  interest  whether 

7 


8       DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

a  machine  intended  for  one  light  can  or  cannot  produce 
two  in  the  same  circuit,  and  if  not,  why  not ;  whether  a 
machine  such  as  the  Wallace-Farmer,  intended  as  it  is  for 
many  lights,  will  give  economical  results  when  used  for 
one;  and  so  on.  It  is  clear  that  the  attempt  to  examine 
all  separate  combinations  of  so  many  variables  would  be 
hopeless,  and  that  the  work  must  be  systematized. 

The  mechanical  energy  communicated  by  the  steam  en- 
gine or 'other  motor  is  not  immediately  converted  into  the 
energy  of  heat,  but  is  first  converted  into  the  energy  of 
an  electric  current  in  a  conducting  circuit;  of  this  a 
portion  only  becomes  localized  as  heat  between  the  car- 
bons of  the  electric  arc ;  and  of  this  again  a  part  only  be- 
comes sensible  to  the  eye  as  light.  The  whole  of  what  we 
need  to  know  may  be  more  easily  ascertained  and  more 
shortly  expressed  if  the  inquiry  is  divided  into  two  parts : 
(a)  What  current  will  a  machine  produce  under  various 
conditions  of  circuit,  and  at  what  expenditure  of  mechani- 
cal power  ?  (b)  Having  given  the  electric  conditions  under 
which  the  arc  is  placed,  no  matter  how  these  conditions 
are  produced,  what  light  will  be  obtained  therefrom  ? 
Parts  of  the  subject  have  been  treated  more  or  less  in  this 
sense  by  Edlund  (Pogg.  Ann.,  1867  and  1868),  Houston 
and  Thomson  in  America,  Mascart  (Journal  de  Physique, 
March,  1878),  Abney  (Proceedings  of  the  Royal  Society, 
1878),  Trowbridge  (Philosophical  Magazine,  March,  1879), 
Schwendler  (Report  on  Electric  Light  Experiments),  etc., 
but  not  so  completely  that  nothing  remains  to  be  done ; 
nor  does  the  author  doubt  that  a  great  deal  of  information 
is  in  the  hands  of  makers  of  machines,  which  they  have 
not  thought  it  necessary  to  make  known.  The  present 


ON   ELECTRIC    LIGHTING.  9 

communication  is  limited  to  an  account  of  some  experi- 
ments on  the  production  of  currents  by  a  Siemens  medium- 
sized  machine;  that  is,  the  machine  which  is  advertised  to 
produce  a  light  of  6,000  candles  by  an  expenditure  of  3J 
horse  power. 

All  the  machines  for  converting  mechanical  power  into 
an  electric  current  consist  ultimately  of  a  conducting  wire 
moving  in  a  magnetic  field;  and  approximately  the  elec- 
tromotive force  of  the  machine  will  be  proportional  to  the 
velocity  with  which  the  circuit  moves  through  the  field, 
and  to  the  intensity  of  the  field.  In  general  the  intensity 
of  the  field  is  not  constant;  and  in  such  machines  as  the 
Siemens  and  the  ordinary  Gramme  machine  it  may  be  re- 
garded as  a  function  of  the  current  passing.  We  must 
learn  what  this  function  is  for  the  machine  in  question ; 
or — which  comes  to  exactly  the  same  thing,  and  is  better 
so  long  as  the  facts  are  merely  the  result  of  experiment — 
we  must  construct  a  curve  in  which  the  abscissae  represent 
the  intensities  of  currents  passing,  and  the  ordinates  the 
corresponding  electromotive  forces  for  a  given  speed  of 
rotation.  But  the  power  of  a  current,  that  is,  its  energy 
per  second,  is  the  product  of  the  electromotive  force  and 
the  intensity,  or,  in  the  case  of  the  curve,  the  product  of 
the  ordinate  and  the  abscissa;  this  is  in  all  cases  less  than 
the  power  required  to  drive  the  machine,  and  the  ratio  be- 
tween the  two  may  fairly  be  called  the  efficiency  of  the 
machine. 

The  object  of  the  inquiry  may  perhaps  be  made  clearer 
by  an  illustration.  Consider  the  case  of  a  pump  forcing 
water  through  a  pipe  against  friction;  then  the  electric 
current  corresponds  to  the  volume  of  water  passing  per 


10    DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

second,  and  the  electromotive  force  to  the  difference  of 
pressure  on  the  two  sides  of  the  pump;  and  just  as  the 
product  of  pressure  and  volume  per  second  is  power,  so 
the  product  of  electromotive  force  and  current  is  power, 
which  is  directly  comparable  with  the  power  expended  in 
driving  the  machine  or  the  pump,,  as  the  case  may  be. 
The  peculiarity  of  the  so-called  dynamo-electric  machine 
lies  in  this,  that  what  corresponds  to  the  difference  of 
pressure  (the  electromotive  force)  depends  directly  on 
what  corresponds  to  the  volume  passing  (the  current). 

Each  experiment  requires  the  determination  of  the 
speed,  the  driving  power,  the  resistances  in  circuit,  and 
the  current  passing;  or  of  the  difference  of  potential  be- 
tween the  two  ends  of  a  known  resistance  of  the  circuit. 

The  apparatus  employed  by  the  author  was  arranged, 
not  alone  with  an  aim  to  accuracy,  but  in  part  to  make  use 
of  such  instruments  as  he  happened  to  possess  or  could 
easily  construct,  and  in  part  with  a  view  to  ready  erection 
and  transportation.  Much  more  accurate  results  may  be 
obtained  by  any  one  who  will  arrange  apparatus  with  a  single 
aim  to  attain  the  greatest  accuracy  possible.  The  author's 
apparatus  will,  however,  be  briefly  described,  that  others 
may  form  their  own  opinion  of  the  importance  of  the  va- 
rious sources  of  error. 

The  speed  counter  was  that  supplied  with  the  electric 
machine. 

Concerning  the  steam  engine  nothing  need  be  said,  save 
that  its  speed  was  maintained  very  constant  by  means  of  a 
governor,  shown  in  Fig.  1,  specially  arranged  for  great 
sensibility.  By  placing  the  joint  A  above  the  joint  B,  in- 
stead of  below  it,  as  in  Porter's  governor,  any  degree  of 


ON   ELECTRIC   LIGHTING. 


11 


FIG.  1.— GOVERNOR. 


sensibility  up  to  instability  may  be  obtained.     The  speed 
was  varied  by  means  of  a  weight  and  a  spring  attached  to 
a  lever  on   the  throttle  valve 
spindle.       The    ungainly    ap- 
pearance of  this  governor  could 
easily  be  remedied  by  any  one 
proposing  to  manufacture  it. 

The  power  is  transmitted 
from  the  engine  to  a  counter- 
shaft by  means  of  a  strap,  and 
by  a  second  strap  from  the 
countershaft  to  the  pulley  of 
the  electric  machine.  On  this 
second  strap  is  the  dynamom- 
eter shown  in  Fig.  2. 

This  dynamometer  has  for  some  time  been  used  by 
Messrs.  Siemens,  and  was  also  used  by  Mr.  Schwendler;  its 
invention  is  due  to  Herr  von  Hefner- Alteneck.  A  is  the 
driving  pulley ;  B  the  pulley  of  the  electric  machine ;  C  C 
are  a  pair  of  loose  pulleys  between  which  the  strap  passes ; 
these  are  carried  in  a  double  triangular  frame,  which  can 
turn  about  a  bar  D.  This  bar  might  form  part  of  a  per- 
manent structure ;  but  in  order  to  place  the  dynamometer 
readily  on  any  strap,  the  bar  was  in  this  case  provided  with 
eyes  at  either  end,  and  secured  in  position  by  six  or  eight 
ropes.  This  plan  answers  well,  as  there  is  very  little  stress 
on  the  bar.  Immediately  above  the  pulleys  C  C  a  cord 
leads  from  the  frame  through  a  Salter  spring  balance  over 
snatch  blocks  to  a  back  balance  weight ;  the  tension  of  this 
cord  is  read  on  the  spring  balance.  At  first  the  spring 
balance  was  omitted,  and  the  weight  at  the  end  of  the  cord 


12     DYNAMO  MACHINERY  AND   ALLIED   SUBJECTS. 


ON  ELECTKIC  LIGHTING.  13 

was  observed  ;  but  the  friction  of  the  snatch  block  pulleys 
was  found  objectionable.  The  pulley  frame  carries  a 
pointer,  which  is  adjusted  so  as  to  coincide  with  a  datum 
mark  when  the  line  A  B  bisects  the  distance  between  the 
loose  pulleys.  Let  W  be  the  tension  of  the  cord  required 
to  bring  the  pulley  frame  to  its  standard  position  when  no 
work  is  being  transmitted,  W"  the  tension  which  is 
required  to  bring  the  pointer  back  to  the  datum  mark  when 
an  observation  is  made,  and  let  W  =  W  —  W".  Let 
T',  T"  be  the  tensions  on  the  tight  and  slack  halves  of  the 
strap;  Rl9  #2,  r  the  radii  of  the  pulleys  A,  B  and  C,  plus 
half  the  thickness  of  the  strap;  c,,  ea  the  distances  AJ, 
J  B;  2d  the  distance  apart  of  the  centres  (7,  C;  al}  #2  the 
inclinations  of  the  two  parts  of  the  strap,  on  either  side  of 
C,  C,  to  the  line  A  B.  Then 

(Tf  -  T")(sin  a,  +  sin  a,)  =  W; 

R,  +  r  -  d  .    d  (R,  +  r  -  dV 
and  sm  a.  =  —  —  ---  \-  —  —  —  -  --    , 

cl  2<?,\         cl         I  ' 

R,  +  r  -  d  ,    d  fR,  +  r  - 

-- 


very  nearly. 

The  value  of  T'  —  T"  and  the  velocity  of  rotation  of  the 
electric  machine  being  known,  the  power  received  by  it  is 
readily  obtained,  expressed  in  gram-centimetres  per  second. 
Multiplying  by  981,  the  value  of  gravity  in  centimetres 
and  seconds,  the  power  is  then  expressed  in  ergs*  per 

*  The  dyne  is  the  force  which  will  in  one  second  impart  to  one  gram  a 
velocity  of  one  centimetre  per  second,  and  an  erg  is.  the  work  done  by  a  dyne 
working  through  a  centimetre;  a  horse  power  may  be  taken  as  three-quarters 


14    DYXAMO  MACHINERY   AND  ALLIED  SrBJBCTS. 


is  ready  for  comparison  with  the  results  of  the 
electrical  experiments. 

As  already  stated,  the  dynamo-electric  machine  in  the 
present  case  was  a  Siemens  medium  size;  the  armature  coil 
has  fifty-ax  divisions,  and  the  brushes  are  single,  not 
divided,  that  is,  each  brash  is  in  connection  with  one 
segment  of  the  commutator  at  any  instant 

The  leading  wire  is  100  yards  of  Siemens  No.  90,  con- 
sisting of  seven  copper  wires,  insulated  with  tape  and  India 
rubber,  and  having  a  diameter  of  about  9J  mm. 

The  method  of  determining  the  current  is  shown  in  the 
diagram,  Fig.  3.  The  current  is  conveyed  from  the 
machine  A  through  a  set  of  coils  of  brass  wire  r,  and  in 
some  eases  through  a  resistance  coil  placed  in  a  calorimeter 
B,  and  so  back  to  the  machine,  the  connections  being 
made  through  cups  of  mercury  excavated  in  a  piece  of 
wood  />.  The  current  passing  may  be  ascertained  by  the 
heating  of  the  calorimeter,  or  by  measuring  the  difference 
of  potential  at  the  extremities  of  the  resistance  r.  all  the 
resistances  of  the  circuit  being  supposed  known.  This 
difference  of  potential  could  of  course  be  very  easily 
measured  by  "means  of  a  quadrant  electrometer;  but,  as 
the  instrument  had  to  be  frequently  removed,  a  galva- 
nometer appeared  more  convenient.  The  two  points  to  be 
measured  are  connected  to  the  ends  of  two  series  of  resist- 
ance coils  a.  b.  The  galvanometer  G  is  placed  in  a  second 
derived  current,  passing  from  a  junction  in  a  b  through  a 
battery  H9  then  through  a  set  of  high  resistances  J  for 

being  M"erg».    Bee  Report  of  fW  Brit. 
G.  S-  System  of  Unto,"  pvJbfiaked  by  tfce 


OS  KLECTB1C  UGHTIXG. 


15 


_« 

:  •  ~" 


16     DYNAMO  MACHINERY    AND   ALLIED   SUBJECTS. 

adjusting  sensibility,  a  reversing  key  K,  the  galvanometer 
6r,  the  reversing  key  K  again,  and  so  to  the  other  extremity 
of  b.  The  electromotive  force  is  ascertained  by  adjusting 
the  resistance  b  so  that  the  deflection  of  the  galvanometer 
is  nil. 

The  resistance  coils  c  comprise  ten  coils  of  common 
brass  wire,  each  wound  round  a  couple  of  wooden  uprights 
driven  into  a  baseboard  common  to  the  set;  each  wire  is 
about  60  metres  long,  and  of  No.  17  Birmingham  wire 
gauge  (.06  inch  or  1-J  mm.  diameter),  weighing  about  14.6 
grams  per  metre.  Each  terminal  is  connected  to  a  cup  of 
mercury  excavated  in  the  baseboard,  so  that  the  coils  can 
be  placed  in  series  or  in  parallel  circuit  at  pleasure.  The 
resistance  of  each  coil  being  about  3  ohms,  this  set  may  be 
arranged  to  give  resistances  varying  from  0.3  to  30  ohms. 

The  calorimeter  B  is  a  Siemens  pyrometer  with  the  top 
scale  removed;  a  resistance  coil  of  uncovered  German 
silver  wire  nearly  2  m.  long,  1J  mm.  in  diameter,  and 
having  a  resistance  of  about  0.2  ohm,  is  suspended  within 
it  from  an  ebonite  cover,  which  also  carries  a  little  brass 
stirrer,  and  the  calorimeter  is  filled  with  water  to  a  level 
determined  by  the  mark  of  a  scriber.  It  was  of  course 
necessary  to  know  the  capacity  of  the  calorimeter  for  heat. 
It  was  filled  with  warm  water  up  to  the  mark,  and  the  coil 
placed  in  position ;  120  grams  of  water  were  then  with- 
drawn, and  the  temperature  of  the  calorimeter  was  ob- 
served to  be  58.8°  C. ;  after  the  lapse  of  one  minute  it  was 
58.3°  C.;  after  a  second  minute  57.9°  0.;  120  grams  of 
cold  water,  temperature  13.3°  C.,  were  then  suddenly  in- 
troduced through  a  hole  in  the  ebonite  cover,  and  it  was 
found  that,  two  minutes  after  the  reading  of  57.9°  C.,  the 


ON  ELECTEIC   LIGHTING.  17 

temperature  was  50.0°  C.;  hence  we  may  infer  that  the 
capacity  of  the  calorimeter  is  equal  to  that  of  740  grams  of 
water.  Two  similar  experiments  at  lower  temperatures 
gave  respectively  the  numbers  749  and  750.  Estimating 
the  capacity  from  the  weight  of  the  copper  cylinder  sup- 
plied with  the  pyrometer,  it  should  be  747,  to  which  must 
be  added  the  capacity  of  the  German  silver  wire  and 
stirrer.  Taking  everything  into  consideration,  750  grams 
may  be  assumed  as  the  most  probable  result. 

The  resistance  coils  a,  b  are  of  German  silver,  made  by 
Messrs.  Elliott  Brothers ;  they  are  on  the  binary  scale  from 
-J  ohm  to  1,024  ohms.  Separate  coils  were  used,  instead  of 
a  regular  resistance  box,  because  they  were  more  readily 
applicable  to  any  other  purpose  for  which  they  might  be 
required;  and  the  binary  scale  was  adopted,  because  the 
coils  could  at  once  be  used  as  conductivity  coils  in  parallel 
circuit,  also  on  the  binary  scale.  Each  coil  as  supplied 
terminated  in  two  stout  copper  legs ;  these  were  fitted  with 
cups  of  india  rubber  tubing  for  mercury,  whereby  any  con- 
nections whatever  could  readily  be  made.  This  arrange- 
ment, though  rude,  was  very  convenient,  and  perhaps  even 
safer  from  error  than  a  box  with  brass  plugs  to  make  the 
connections.  By  a  slight  alteration  of  the  connections  the 
whole  was  instantly  available  as  a  Wheatstone  bridge  to 
determine  resistances. 

The  battery  H  is  a  single  element  of  Daniell's  battery, 
in  which  the  sulphate  of  zinc  solution  floats  on  the  sul- 
phate of  copper;  its  electromotive  force  is  assumed  to  be 
|  volt. 

The  resistances  J  added  in  the  battery  circuit  are  pencil 
lines  on  glass,  such  as  are  described  in  the  Philosophical 


18   DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

Magazine  of  February,  1879.  Three  were  used,  giving  a 
range  of  sensibility  approximately  in  the  proportions  1,  25, 
170,  700— the  last  figure  being  when  all  were  short  cir- 
cuited ;  they  are  very  useful  in  adjusting  the  resistance  b 
so  as  to  give  no  deflection  of  the  galvanometer. 

The  reversing  key  K  belongs  to  Sir  W.  Thomson's  elec- 
trometer, and  is  quite  suitable  when  high  resistances  and 
nil  methods  are  used. 

The  galvanometer  G  is  far  more  sensitive  than  necessary, 
and  has  a  resistance  of  7,000  ohms. 

Preliminary  to  experiments  on  the  current,  determina- 
tions of  resistances  were  made.  The  resistance  of  each 
brass  coil  c  was  first  determined,  to  afford  the  means  of. 
calculating  the  value  of  this  resistance  in  any  subsequent 
experiment.  When  the  ten  coils  were  coupled  in  parallel 
circuit,  the  calculated  resistance  was  0.29  ohm,  while  0.292 
was  obtained  by  direct  measurement.  The  leading  wire 
was  then  examined;  the  further  ends  being  disconnected, 
the  insulation  resistance  was  found  to  be  over  60,000  ohms ; 
how  much  over,  it  was  immaterial  to  learn.  When  the 
ends  of  the  wire  were  connected,  the  resistance  was  found 
to  be  0.129  ohm.  The  resistances  in  the  dynamo-electric 
machine  A  were  found  to  be  as  follows  when  cold:  magnet 
coils,  0.156  and  0.152  respectively;  armature  coil,  0.324; 
total,  0.632  ohm.  Direct  examination  was  made  of  the 
whole  machine  in  eight  positions  of  the  commutator, 
giving  0.643  ohm,  with  a  maximum  variation  of  0.6  per 
cent,  from  the  mean.  After  running  the  machine  for 
some  time  the  resistance  was  found  to  be  0.683,  an  increase 
which  would  be  accounted  for  by  a  rise  of  temperature  of 
12°  0.  or  thereabouts.  The  resistance  of  the  calorimeter 


Otf  ELECTKIC   LIGHTING.  19 

B  is  0.20  ohm,  without  its  leading  wire,  which  may  be 
taken  as  0.01.  We  have  then  in  circuit  three  resistances 
which  must  be  considered:  (1)  The  resistance  of  the 
machine  A  and  leading  wire,  assumed  throughout  as 
together  0.81  ohm,  and  denoted  bye,;  (2)  the  resistance 
of  the  brass  coils  c,  calculated  from  the  several  determina- 
tions, with  the  addition  of  0.02  ohm,  the  resistance  of  the 
leading  wire,  and  denoted  by  c2;  (3)  when  present,  the 
resistance  of  the  calorimeter  B  and  leading  wire,  denoted 
by  c3. 

Two  approximate  corrections  were  employed,  and  should 
be  detailed.  The  first  is  the  correction  for  the  consider- 
able heating  of  the  resistance  coils  c.  These  were  arranged 
in  two  sets  of  five  each,  five  being  in  parallel  circuit,  and 
the  two  sets  in  series.  The  current  from  the  machine, 
being  about  7.4  webers  in  each  wire,  was  passed  for  three 
or  four  minutes;  the  circuit  was  then  broken,  and  the 
resistance  c2  was  determined  within  one  second  of  breaking 
circuit,  when  it  was  found  to  be  about  5  per  cent,  greater 
than  when  cold.  As  the  resistance  was  falling,  the  follow- 
ing was  adopted  as  a  rule  of  correction :  square  the  current 
in  a  single  wire,  and  increase  the  resistance  c2  by  ^  per 
cent,  for  every  unit  in  the  square.  The  second  correction 
is  due  to  the  fact  that  the  calorimeter  was  losing  heat  all 
the  time  it  was  being  used.  It  was  assumed  that  it  loses 
0.01°  C.  per  minute  for  every  1°  0.  by  which  the  tempera- 
ture of  the  calorimeter  exceeds  that  of  the  air;  this  cor- 
rection is  of  course  based  on  the  experiment  already 
mentioned. 

The  method  of  calculation  may  now  be  explained: — 


20     DYNAMO  MACHINERY   AND   ALLIED   SUBJECTS. 
R  is  the  total  resistance  of  the  circuit  in  ohms,  equal  to 

CI  +  C.H-CS; 

Q  is  the  current  passing  in  webers; 
E  is  the  electromotive  force  round  the  circuit  in  volts; 
Wl  is  the  work  per  second  converted  into  heat  in  the 
circuit,  as  determined  by  the  galvanometer,  measured 
in  erg-tens  per  second  ; 
Wa   is   the   work    per    second    as    determined    by  the 

calorimeter; 

TF3  is  the  work  per  second  as  determined  by  the  dyna- 
mometer, less  the  power  required  to  drive  the  machine 
when  the  circuit  is  open; 
HP  is  the  equivalent  of  W3  in  horse  power; 
n  is  the   number   of  revolutions   per    minute    of  the 

armature. 

As  already  mentioned,  the  standard  resistance  coils  a,  b 
are  adjusted  in  each  experiment  so  that  the  galvanometer 
gives  no  deflection,  and  the  value  of  b  is  then  noted. 
The  values  of  clt  c2,  c3  are  known  from  previous  observa- 
tions. Then 


_ 
~' 


E  =  Q  x  R, 
W,  =  E  x  Q, 


mechanical  equivalent  of  heat 
generated  per  second  in  calorimeter. 


The  results  of  the  experiments  are  given  in  the  accom- 
panying table. 


ON   ELECTRIC    LIGHTING. 


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22     DYNAMO   MACHINERY   AND    ALLIED    SUBJECTS. 

A  power  of  0.21  erg-ten,  or  0.28  horse  power,  was  re- 
quired to  drive  the  machine  at  720  revolutions  on  open 
circuit.  An  examination  of  the  table  shows  that  the 
efficiency  of  the  machine  is  about  90  per  cent.,  exclusive 
of  friction.  Comparing  experiments  11  and  13,  and  also 
the  last  four  experiments,  it  is  seen  that  the  electromotive 
force  is  proportional  to  the  speed  of  rotation  within  the 
errors  of  observation.  Experiments  14,  15,  and  16  were 
intended  to  ascertain  the  effect  of  displacing  the  com- 
mutator brushes. 

The  principal  object  of  the  experiments  was  to  ascertain 
how  the  electromotive  force  depended  on  the  current. 
This  relation  is  represented  by  the  curve  shown  in  Fig.  4, 


Curves  of  Force  and  Current 
Speed  720  revolutions  per  minute 


40  50  60  TOWebcrs 

FIG.  4. — CURVE  OF  FORCE  AND  CURRENT. 

in  which  the  abscissae  represent  the  currents  flowing,  or  the 
values  of  Q  in  the  table,  and  the  ord  mates  the  electro- 
motive forces,  or  the  values  of  E  reduced  to  a  speed  of  720 
revolutions  per  minute.  The  curve  may  also  be  taken  to 
represent  the  intensity  of  the  magnetic  field,  It  will  be. 


ON   ELECTRIC   LIGHTING.  23 

remarked  that  there  is  a  point  of  inflection  in  the  curve 
somewhere  near  the  origin.  The  experiments  1  to  5  indi- 
cate that  this  is  the  true  form  of  the  curve,  and  it  is  con- 
firmed in  a  remarkable  manner  by  a  special  experiment. 
A  resistance  intermediate  between  5J  and  4  (experiments 
3  and  4)  was  used  in  circuit,  and  E  and  Q  were  determined 
in  two  different  ways:  first,  by  starting  with  an  open 
circuit,  which  was  then  closed ;  secondly,  by  starting  with 
a  portion  of  the  resistance  short  circuited,  and  a  very 
powerful  current  passing,  and  then  breaking  the  short 
circuit.  It  was  found  that '  E  and  Q  were  four  times  as 
great  in  the  latter  case  as  in  the  former.  Unfortunately 
the  numbers  are  not  sufficiently  accurate  to  be  given,  as 
the  solutions  of  the  standard  battery  had  become  mixed. 

The  curve  really  gives  a  great  deal  more  information 
than  appears  at  first  sight.  It  will  determine  what  current 
will  flow  at  any  given  speed  of  rotation  of  the  machine, 
and  under  any  conditions  of  the  circuit,  whether  of  resist- 
ances or  of  opposed  electromotive  forces.  It  will  also  give 
very  approximate  indications  of  the  corresponding  curve 
for  other  machines  of  the  same  configuration,  but  in  which 
the  number  of  times  the  wire  passes  round  the  electro- 
magnet or  the  armature  is  different. 

It  will  be  well  to  compare  these  results  with  those 
obtained  by  others.  M.  Mascart  worked  on  a  Gramme 
machine  with  comparatively  low  currents:  he  represents 
his  results  approximately  by  the  formula 

E  =  n  (a  +  b  Q), 

where  a  and  b  are  constants.  This  corresponds  to  the 
rapidly  rising  part  of  the  curve  in  Fig.  4,  Mr,  Trowbridge 


24     DYNAMO   MACHINEBY   AND   ALLIED   SUBJECTS. 

with  a  Siemens  machine  obtained  a  maximum  efficiency  of 
76  per  cent.,  and  states  that  the  machine  was  running 
below  its  normal  velocity.  Mr.  Schwendler's  results,  when 
fully  published,  will  probably  be  found  to  be  the  most 
complete  and  most  accurate  existing.  In  the  precis  he 
states  that  the  loss  of  power  with  a  Siemens  machine  in 
producing  currents  of  over  20  webers  is  12  per  cent. 
Now,  taking  the  author's  experiments  4  to  19,  the  mean 
value  of  W,  is  3.027  erg-tens  and  of  W3  3.304;  adding  to 
the  latter  0.21,  the  power  required  to  drive  the  machine 
when  no  current  passes,  it  appears  that  13.8  per  cent,  of 
the  power  applied  is  wasted.  Again,  taking  experiments 
4,  6,  8,  10,  and  12,  the  mean  value  of  JF2  is  2.888  erg-tens 
and  of  Ws  3.076,  indicating  a  waste  of  power  amounting  to 
12  per  cent.  Of  this,  as  already  stated,  0.21  erg-ten,  or 
0.28  horse  power,  is  accounted  for  by  friction  of  the 
journals  and  commutator  brush;  the  remainder  is  ex- 
pended in  local  currents,  or  by  loss  of  kinetic  energy  of 
current  when  sparks  occur  at  the  commutator. 

According  to  Weber's  theory  of  induced  magnetism,  as 
set  forth  in  Maxwell's  "Electricity  and  Magnetism," 
vol.  II.,  if  X  be  the  magnetizing  force  and  /  the  intensity 
of  magnetization, 

2      X 
1=  —a  — ,  until  X rises  to  the  value  b, 

6         o 

and          I= 

where  a  and  b  are  constants.  We  should  naturally  expect 
that  a  similar  formula  would  be  approximately  applicable 
to  dynamo-electric  machines. 


ON    ELECTKIC   LIGHTING.  25 

In  the  present  experiments,  let  1  be  the  electromotive 
force,  X  the  current  passing,  and  assume  a  to  be  60  and  b 
to  be  15;  we  then  obtain  results  not  far  from  those  of 
experiment.  The  capacity  of  any  continuous  current 
machine  may  thus  be  shortly  stated  by  giving  the  values  of 
a  and  b;  or,  which  comes  to  the  same  thing,  by  stating  the 
electromotive  force  at  a  given  speed  when  the  -current  is 
as  great  as  possible,  and  also  the  total  resistance  through 
which  the  machine  will  exert  an  electromotive  force  two- 
thirds  of  this  greatest  electromotive  force.  To  this  should 
be  added  a  statement  of  the  resistance  of  the  machine, 
and  of  the  power  it  absorbs,  with  known  conditions  of  the 
circuit. 

The  author  has  not  yet  tried  any  quantitative  experi- 
ments with  the  electric  light,  but  hopes  shortly  to  do  so. 
In  the  meantime  he  would  remark  that,  as  the  lamp  is 
usually  adjusted,  only  half  the  energy  of  the  current 
appears  in  the  arc,  or  44  per  cent,  of  the  energy  trans- 
mitted to  the  machine  by  the  strap. 

In  conclusion  the  author  would  express  the  obligation 
he  is  under  to  Messrs.  Chance  Brothers  &  Co.,  on  account 
of  the  facilities  he  has  enjoyed  for  making  these  experi- 
ments at  their  works.  It  may  be  mentioned  that  one 
principal  object  of  the  research  of  which  this  is  a  beginning 
is  to  obtain  a  minute  knowledge  of  the  electric  light,  with 
a  view  to  lighthouse  illumination. 


26      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


ON  ELECTRIC  LIGHTING. 


SECOND   PAPER. 


Dynamo-Electric  Machines. — Since  the  date  of  the 
author's  former  paper  in  April,  1879,  other  observers  have 
published  the  results  of  experiments  similar  to  those  de- 
scribed by  him.  It  may  be  well  to  exhibit  some  of  these 


'0  Webers 


FIG.  5.— CURVES  OF  ELECTROMOTIVE  FORCE  AND  CURRENT  OP  SIEMENS  MEDIUM 
SIZE  MACHINE. 

results  reduced  to  the  form  he  has  adopted,  namely,  a  curve, 
such  as  that  previously  shown  in  Fig.  4  of  the  preceding 
paper,  and  now  reproduced,  with  slight  alterations,  in  Fig. 
5.  Here  any  abscissa  represents  a  current  passing  through 
the  dynamo-electric  machine,  and  the  corresponding  ordi- 


ON   ELECTRIC   LIGHTING.  27 

nate  represents  the  electromotive  force  of  the  machine  for 
a  certain  speed  of  revolution,  when  that  current  is  passing 
through  it.  It  will  be  found  (1)  that  with  varying  speed 
the  ordinate,  or  electromotive  force,  corresponding  to  any 
abscissa  or  current  is  proportional  to  the  speed;  (2)  that 
the  electromotive  force  does  not  increase  indefinitely  with 
increasing  current,  but  that  the  curve  approaches  an  asymp- 
tote; (3)  that  the  earlier  part  of  the  curve  is,  roughly 
speaking,  a  straight  line,  until  the  current  attains  a  certain 
value,  and  that  at  that  point  the  electromotive  force  has 
reached  about  two-thirds  of  its  maximum  value.  When  the 
current  is  such  that  the  electromotive  force  is  not  more 
than  two-thirds  of  its  maximum,  a  very  small  change  in  the 
resistance  with  speed  of  engine  constant,  or  in  the  speed  of 
the  engine  with  resistance  constant,  causes  a  great  change 
in  the  current.  For  this  reason  the  greatest  of  these  cur- 
rents, which  is  that  corresponding  to  the  point  where  the 
curve  breaks  away  from  a  straight  line,  and  which  is  the 
same  for  all  speeds  of  revolution,  since  the  curves  for  dif- 
ferent speeds  differ  only  in  the  scale  of  ordinates,  may  be 
called  the  "  critical  current  "  of  the  machine.  The  effect 
of  a  change  of  speed  is  exhibited  in  Fig.  5,  where  the  lower 
dotted  line  represents  the  curve  for  a  speed  of  660  revolu- 
tions per  minute,  instead  of  720.  The  resistance,  varying  as 

electromotive  force  .       .        -,     41       i  4.  ,-,     v       ^  n 

— ,  is  given  by  the  slope  of  the  line  0  P. 
current 

But  since  the  resistance  is  constant,  the  slope  of  this  line 
must  be  constant;  and  it  will  be  seen  that  it  cuts  the 
upper  curve  at  a  point  corresponding  to  a  current  of  15 
webers,  and  the  lower  at  a  point  corresponding  to  a  current 
of  5  webers  only. 


28      DYNAMO   MACHINERY   AND   ALLIED    SUBJECTS. 


In  Germany,  Auerbach  and  Meyer  (  Wiedemann  Annalen, 
Nov.,  1879)  have  experimented  fully  on  a  Gramme  machine 
at  various  speeds,  and  with  various  external  resistances. 
The  resistance  of  the  machine  was  0.97  ohm.  Their  results 
are  summarized  in  a  table  at  the  end  of  their  paper,  which 
gives  the  current  passing,  with  resistances  in  circuit  from 
1.75  to  200  Siemens  units,  and  at  speeds  from  20  to  800 
revolutions  per  minute.  In  the  accompanying  diagram, 
Fig.  6,  the  curve  G  expresses  the  relation  between  electro- 


Current 


70  Webers 

FIG.  6.— CURVES  OP  ELECTROMOTIVE  FORCE  AND  CURRENT  :  GRAMME  MACHINE, 
G;  SIEMENS  MEDIUM,  SM;  SIEMENS  SMALLEST,  Ss. 

motive  force  and  current,  as  deduced  from  some  of  their 
observations;  the  points  marked  are  plotted  from  their 
table,  making  allowance,  where  necessary,  for  difference  in 
speed.  The  curve,  as  actually  constructed,  is  for  a  speed 
of  800  revolutions :  at  this  speed  it  will  be  seen  that  the 
maximum  electromotive  force  is  about  76  volts;  and  the 
critical  current,  corresponding  to  a  force  of  about  51  volts, 
is  6.5  webers,  with  a  total  resistance  of  7.8  ohms.  Up  to 
this  point  there  will  be  great  instability,  exactly  as  was  the 


ON   ELECTRIC   LIGHTING.  29 

case  in  the  Siemens  machine  examined  by  the  author,  where 
the  resistance  was  4  ohms  and  the  speed  720  revolutions. 

The  results  of  an  elaborate  series  of  experiments  on  cer- 
tain dynamo-electric  machines  have  recently  been  presented 
to  the  Koyal  Society  by  Dr.  Siemens.  One  of  the  machines 
examined  was  an  ordinary  medium  sized  machine,  substan- 
tially similar  to  that  tried  by  the  author  in  1879.  It  is 
described  as  having  24  divisions  of  the  commutator;  336 
coils  on  the  armature,  with  a  resistance  of  0.4014  Siemens 
unit;  and  512  coils  on  the  magnets,  with  a  resistance  of 
0.3065:  making  a  total  resistance  of  0.7079  Siemens  unit 
=  0.6654  ohm.  The  curve  8m,  Fig.  6,  gives  the  relation 
of  electromotive  force  and  current,  reduced  to  a  speed  of 
700  revolutions  per  minute,  the  actual  speeds  ranging  from 
450  to  800  revolutions.  The  maximum  electromotive 
force  appears  to  be  probably  76  volts,  and  the  critical 
current  15  webers;  which  is  the  same  as  in  the  author's 
first  experiments  on  a  similar  machine. 

In  the  summer  of  1879  the  author  examined  a  Siemens 
machine  of  the  smallest  size.  This  machine  is  generally 
sold  as  an  exciter*  for  their  alternate  current  machine.  It 
has  an  internal  resistance  of  0.74  ohm,  of  which  0.395  is  in 
the  armature  or  helix.  The  machine  is  marked  to  run  at 
1,130  revolutions  per  minute.  The  following  Table  II. 
gives,  for  a  speed  of  1,000  revolutions,  the  total  resistance, 
current,  electromotive  force  and  horse  power  developed  as 
current.  The  horse  power  expended  was  not  determined. 

The  curve  S  s,  Fig.  6,  gives  as  usual  the  relations  olelec- 
tromotive  force  and  current.  From  this  curve  it  will  be  seen 
that  the  critical  current  is  11.2  webers,  and  the  maximum 
electromotive  force,  at  the  speed  of  1,000  revolutions,  is 


30     DYNAMO  MACHINERY  AND   ALLIED   SUBJECTS. 


TABLE    II.— EXPERIMENTS  ON  SMALLEST-SIZED    SIEMENS    DYNAMO- 
ELECTRIC  MACHINE. 


Resistance. 

Electric  Current. 

Electromotive 
Force. 

Horse  Power  Devel- 
oped as  Current. 

2.634  ohms. 

5.10  webers. 

13.2  volts. 

0.09H.P. 

2  221 

12.15 

27.0 

0.44 

1.967 

' 

17.0 

33.6 

0.76 

1.784 

20.4 

36.4 

0.99 

1.668 

22.3 

37.2 

1.11 

1.579 

23.2 

36.6 

1.14 

1.503 

25.6 

39.3 

1.34 

1.440 

27.8 

40.0 

1.49     " 

1.145 

36.2         " 

41.5 

2.00     " 

about  42  volts.  The  determinations  for  this  machine  were 
made  in  exactly  the  same  manner  as  in  the  experiments 
on  the  medium  sized  machine,  using  the  galvanometer,  but 
omitting  the  experiment  with  the  calorimeter  (compare 
Table  L,  p.  21). 

The  time  required  to  develop  the  current  in  a  Gramme 
machine  has  been  examined  by  Herwig  (Wiedemann,  June, 
1879).  He  established  the  following  facts  for  the  machine 
he  examined  :  A  reversed  current,  having  an  electromotive 
force  of  0.9  Grove  cell,  sufficed  to  destroy  the  residual 
magnetism  of  the  electromagnets.  If  the  residual  mag- 
netism was  as  far  as  possible  reduced,  it  took  a  much  longer 
time  to  get  up  the  current  than  when  the  machine  was  in 
its  usual  state.  A  longer  time  was  required  to  get  up  the 
current  when  the  external  resistance  was  great  than  when 
it  was  small.  With  ordinary  resistance  the  current  required 
from  f  second  to  1  second  to  attain  its  maximum. 

Brightness  of  the  Electric  Arc. — The  measurement  of 
the  light  emitted  by  an  electric  arc  presents  certain  peculiar 
difficulties.  The  light  itself  is  of  a  different  color  from 


ON   ELECTRIC   LIGHTING.  31 

that  of  a  standard  candle,  in  terms  of  which  it  is  usual  to 
express  luminous  intensities.  The  statement,  without 
qualification,  that  a  certain  electric  lamp  and  machine  give 
a  light  of  a  specified  number  of  candles,  is  therefore  want- 
ing in  definite  meaning.  A  red  light  cannot  with  pro- 
priety be  said  to  be  any  particular  multiple  of  a  green 
light;  nor  can  one  light  which  is  a  mixture  of  colors  be 
said  with  strictness  to  be  a  multiple  of  another,  unless  the 
proportions  of  the  colors  in  the  two  cases  are  the  same. 
Captain  Abney  (Proceedings  of  the  Royal  Society,  March 
7,  1878,  p.  157)  has  given  the  results  of  measurements  of 
the  red,  blue,  and  actinic  light  of  electric  arcs  in  terms  of 
the  red,  blue,  and  actinic  light  of  a  standard  candle.  The 
fact  that  the  electric  light  is  a  very  different  mixture  of 
rays  from  the  light  of  gas  or  of  a  candle  has  long  been 
known,  but  has  been  ignored  in  statements  intended  for 
practical  purposes. 

Again,  the  emission  of  rays  from  the  heated  carbons  and 
arc  is  by  no  means  the  same  in  all  directions.  Determi- 
nations have  been  made  in  Paris  of  the  intensity  in  differ- 
ent directions,  in  particular  cases.  If  the  measurement  is 
made  in  a  horizontal  direction,  a  very  small  obliquity  in 
the  crater  of  the  positive  carbon  will  throw  the  light  much 
more  on  one  side  than  on  the  other,  causing  great  dis- 
cordance in  the  results  obtained. 

If  the  electric  light  be  compared  directly  with  a  stand- 
ard candle,  a  dark  chamber  of  great  length  is  needed — a 
convenience  not  always  attainable.  In  the  experiments 
made  at  the  South  Foreland  by  Dr.  Tyndall  and  Mr. 
Douglass,  an  intermediate  standard  was  employed;  the 
electric  light  was  measured  in  terms  of  a  large  oil  lamp, 


32      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


and  this  latter  was  frequently  compared  with  a  standard 
candle. 

Other  engagements  have  prevented  the  author  from 
fairly  attacking  these  difficulties;  but  since  May,  1879,  he 
has  had  in  occasional  use  a  photometer  with  which  power- 
ful lights  can  be  measured  in  .moderate  space.  This 
photometer  is  shown  in  Figs.  7  and  8,  and  an  enlargement 


5T 

r£U 


^Longitudinal  Section 
FIG.  7.— PHOTOMETER  FOR  POWERFUL  LIGHTS. 


STANDARD 

LIGHT 


of  the  field  piece  in  Fig.  9.  A  convex  lens  A,  of  short 
focus,  forms  an  image  at  B  of  the  powerful  source  of 
light  which  it  is  desired  to  examine.  The  intensity  of 
the  light  from  this  image  will  be  less  than  that  of  the 
actual  source  by  a  calculable  amount;  and  when  the  dis- 
tance of  the  lens  from  the  light  is  suitable,  the  reduction 
is  such  that  the  reduced  light  becomes 
comparable  with  a  candle  or  a  carcel 
lamp.  Diaphragms  C  C  are  arranged 
in  the  cell  which  contains  the  lens,  to 
cut  off  stray  light.  One  of  these  is 
placed  at  the  focus  of  the  lens,  and  has 
a  small  aperture.  It  is  easy  to  see  that 
this  diaphragm  will  cut  off  all  light 
entering  from  a  direction  other  than 
"  that  of  the  source;  so  effectually  does  it 
do  so,  that  observations  may  be  made  in 
broad  daylight  on  any  source  of  light,  if  a  dark  screen 


ON   ELECTRIC   LIGHTING. 


33 


be  placed  behind  it.  The  long  box  E  E,  Fig.  7,  of 
about  7  feet  length,  is  lined  with  black  velvet, — the  old- 
fashioned  dull  velvet,  not  that  now  sold  with  a  finish,  which 
reflects  a  great  deal  of  the  light  incident  at  a  certain  angle. 
This  box  serves  as  a  dark  chamber,  in  which  the  intensity 
of  the  image  formed  by  the  lens  is  compared  with  a  stand- 


FIG.  9.— FIELD  PIECE. 

ard  light,  by  means  of  an  ordinary  Btmsen's  photometer  F, 
sliding  on  a  graduated  bar. 

Mr.  Dallmeyer  kindly  had  the  lens  made  for  the  author: 
he  can  therefore  rely  upon  the  accuracy  of  its  curvature 
and  thickness;  it  is  plano-convex,  the  convex  side  being 
towards  the  source  of  light.  The  curvature  is  exactly  1 
inch  radius  and  the  thickness  is  0.04  inch;  it  is  made  of 
Chance's  hard  crown  glass,  of  which  the  refractive  index  for 
the  D  line  in  the  spectrum  is  1.517.  The  focal  length/  is 
therefore  1.933  inches. 

Let  u  denote  the  distance  of  the  source  of  light  from 


84:      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

the  curved  surface  of  the  lens,  and  v  the  distance  of  the 
image  B  of  the  source  from  the  posterior  focal  plane. 
Neglecting  for  the  moment  loss  by  reflection  at  the  sur- 
face of  the  glass,  the  intensity  of  the  source  is  reduced  by 

the  factor  ( —  1  •  But  — | —  =  -r,  or  v  —  — —•  hence  the  f ac- 
W  v       u      f  u~f 

i    f  Y 

tor  of  reduction  is  I— -- >  •  The  effect  of  absorption   in  so 
\n— fJ 

small  a  thickness  of  very  pure  glass  may  be  neglected ;  but 
the  reflection  at  the  surfaces  will  cause  a  loss  of  8.3  per 
cent.,  which  must  be  allowed  for.  This  percentage  is 
calculated  from  Fresners  formulae,  which  are  certainly 
accurate  for  glasses  of  moderate  refrangibility,  and  for 
moderate  angles  of  incidence. 

Suppose,  for  example,  it  is  required  to  measure  a  light  of 
8,000  candles;  if  it  be  placed  at  a  distance  of  40  inches,  it 
will  be  reduced  in  the  ratio  467  to  1,  and  becomes  a  con- 
veniently measurable  quantity.  By  transmitting  through 
colored  glasses  both  the  light  from  an  electric  lamp  and  that 
from  the  standard,  a  rough  comparison  may  be  made  of 
the  red  or  green  in  the  electric  light  with  the  red  or  green 
in  the  standard. 

A  dispersive  photometer,  in  which  a  lens  is  used  in  a 
somewhat  similar  manner,  is  described  in  Stevenson's 
"Lighthouse  Illumination;"  but  in  that  case  the  lens  is 
not  used  in  combination  with  a  Bunsen  photometer,  nor 
with  any  standard  light.  Messrs.  Ayrton  and  Perry  de- 
scribed a  dispersive  photometer  with  a  concave  lens  at  the 
meeting  of  the  Physical  Society  on  December  13,  1879 
(Proceedings  Physical  Society,  vol.  III.,  p.  184J.  The 
convex  lens  possesses,  however,  an  obvious  advantage  in 


ON   ELECTRIC   LIGHTING.  35 

having  a  real  focus,  at  which  a  diaphragm  to  cut  off  stray 
light  may  be  placed. 

Efficiency  of  the  Electric  Arc. — To  define  the  electrical 
condition  of  an  electric  arc,  two  quantities  must  be  stated 
—the  current  passing,  and  the  difference  of  electric 
potential  at  the  ends  of  the  two  carbons.  Instead  of 
either  one  of  these,  we  may,  if  we  please,  state  the  ratio 
difference  of  potential  d  ^  it  ^  resistance  of  the  arc> 

current 

that  is  to  say,  the  resistance  which  would  replace  the  arc 
without  changing  the  current.  But  such  a  use  of  the  term 
electric  resistance  is  unscientific;  for  Ohm's  law,  on  which 
the  definition  of  electric  resistance -rests,  is  quite  untrue  of 
the  electric  arc;  while  on  the  other  hand,  for  a  given  ma- 
terial of  the  electrodes,  a  given  distance  between  them, 
and  a  given  atmospheric  pressure,  the  difference  of  poten- 
tial on  the  two  sides  of  the  arc  is  approximately  constant. 
The  product  of  the  difference  of  potential  and  the  current 
is  of  course  equal  to  the  work  developed  in  the  arc;  and 
this,  divided  by  the  work  expended  in  driving  the  ma- 
chine, may  be  considered  as  the  efficiency  of  the  whole 
combination.  It  is  a  very  easy  matter  to  measure  these 
quantities.  The  difference  of  potential  on  the  two  sides  of 
the  arc  may  be  measured  by  the  method  given  by  the 
author  in  his  previous  paper,  or  by  an  electrometer,  or  in 
other  ways.  The  current  may  be  measured  by  an  Obach 
galvanometer,  or  by  a  suitable  electro-dynamometer,  or 
best  of  all,  in  the  author's  opinion,  by  passing  the  whole 
current,  on  its  way  to  the  arc,  through  a  very  small  known 
resistance,  which  may  be  regarded  as  a  shunt  for  a  galva- 


36      DYNAMO  MACHINERY  AND   ALLIED   SUBJECTS. 

nometer  of  very  high  resistance,  or  to  the  circuit  of  which 
a  very  high  resistance  has  been  added. 

It  appears  that  with  the  ordinary  carbons,  and  at  ordi- 
nary atmospheric  pressure,  no  arc  can  exist  with  a  less 
difference  of  potential  than  about  20  volts;  and  that  in 
ordinary  work,  with  an  arc  about  i  inch  long,  the  differ- 
ence of  potential  is  from  30  to  50  volts.  Assuming  the 
former  result,  about  20  volts,  for  the  difference  of  poten- 
tial, the  use  of  the  curve  of  electromotive  forces  may  be 
illustrated  by  determining  the  lowest  speed  at  which  a 
given  machine  can  run  and  yet  be  capable  of  producing  a 
short  arc.  Taking  0  as  the  origin  of  co-ordinates,  Fig. 
10,  set  off  upon  the  axis  of  ordinates  the  distance  0  A 


Fio.  10. 


equal  to  20  volts  ;  draw  A  B  to  intersect  at  B  the  negative 
prolongation  of  the  axis  of  abscissae,  so  that  the  ratio  -^= 

may  represent  the  necessary  metallic  resistance  of  the  cir- 
cuit. Through  .the  point  B,  thus  obtained,  draw  a  tangent 
to  the  curve,  touching  it  at  C,  and  cutting  0  A  in  D. 
Then  the  speed  of  the  machine,  corresponding  to  the  par- 
ticular curve  employed,  must  be  diminished  in  the  ratio 


ON    ELECTRIC    LIGHTING. 


37 


--— ,  in  order  that  an  exceedingly  small  arc  may  be  just 

possible. 

The  curve  may  also  be  employed  to  put  into  a  somewhat 
different  form  the  explanation  given  by  Dr.  Siemens,  at 
the  Koyal  Society,  respecting  the  occasional  instability  of 
the  electric  light  as  produced  by  ordinary  dynamo-electric 
machines.  The  operation  of  all  ordinary  regulators  is  to 
part  the  carbons  when  the  current  is  greater  than  a  cer- 
tain amount,  and  to  close  them  when  it  is  less;  initially 
the  carbons  are  in  contact.  Through  the  origin  0,  Fig. 
11,  draw  the  straight  line  OA,  inclined  at  the  angle  repre- 


senting the  resistances  of  the  circuit  other  than  the  arc, 
and  meeting  the  curve  at  A.  The  abscissa  of  the  point  A 
represents  the  current  which  will  pass  if  the  lamp  be  pre- 
vented from  operating.  Let  0  N  represent  the  current  to 
which  the  lamp  is  adjusted;  then  if  the  abscissa  of  A  be 
greater  than  0  N,  the  carbons  will  part.  Through  N  draw 
the  ordinate  B  N9  meeting  the  curve  in  the  point  B;  and 
parallel  to  0  A  draw  a  tangent  CD,  touching  the  curve  at  D. 
If  the  point  B  is  to  the  right  of  D,  or  further  from  the 


38      DYNAMO   MACHINERY    AND   ALLIED    SUBJECTS. 

origin,  the  arc  will  persist;  but  if  B  is  to  the  left  of  D,  or 
nearer  to  the  origin,  the  carbons  will  go  on  parting,  till 
the  current  suddenly  fails  and  the  light  goes  out.  If  B, 
although  to  the  right  of  D,  is  very  near  to  it,  a  very  small 
reduction  in  the  speed  of  the  machine  will  suffice  to  ex- 
tinguish the  light.  Dr.  Siemens  gives  greater  stability  to 
the  light  by  exciting  the  electromagnets  of  the  machine 
by  a  shunt  circuit,  instead  of  by  the  whole  current. 

The  success  of  burning  more  than  one  regulating  lamp 
in  series  depends  on  the  use  in  the  regulator  of  an  electro- 
magnet excited  by  a  high  resistance  wire  connecting  the 
two  opposed  carbons.  The  force  of  this  magnet  will  de- 
pend upon  the  difference  of  potential  in  the  arc,  instead 
of  depending,  as  in  the  ordinary  lamp,  upon  the  current 
passing.  Such  a  shunt  magnet  has  been  employed  in  a 
variety  of  ways.  The  author  has  arranged  it  as  an  attach- 
ment to  an  ordinary  regulator;  the  shunt  magnet  actuates 
a  key,  which  short  circuits  the  magnet  of  the  lamp  when 
the  carbons  are  too  far  parted,  and  so  causes  them  to  close. 

In  conclusion  the  author  ventures  to  remind  engineers 
of  the  following  rule  for  determining  the  efficiency  of  any 
system  of  electric  lighting  in  which  the  electric  arc  is 
used,  the  arc  being  neither  exceptionally  long  nor  excep- 
tionally short:  Measure  the  difference  of  potential  of  the 
arc,  and  also  the  current  passing  through  it,  in  volts  and 
webers  respectively;  then  the  product  of  these  quantities, 
divided  *  by  746,  is  the  horse  power  developed  in  that  arc. 

*  With  respect  to  the  factor  746,  given  above,  the  product  of  difference  of  poten- 
tial and  current  was  power,  which  could  of  course  be  given  as  so  many  foot- 
pounds per  minute;  but  the  number  that  was  got  by  multiplying  webers  and  volts 
together  did  not  give  the  power  in  foot  pounds,  audit  required  a  factor  to  reduce 


ON    ELECTRIC    LIGHTING.  39 

It  is  then  known  that  the  difference  between  the  horse 
power  developed  in  the  arc  and  the  horse  power  expended 
to  drive  the  machine  must  be  absolutely  wasted,  and  has 
been  expended  in  heating  either  the  iron  of  the  machine  or 
the  copper  conducting  wires. 

the  one  to  the  oth  r,  just  as  it  required  a  factor  to  reduce  gramme-centimetres, 
or  any  other  measure  of  power,  to  foot-pounds.  The  factor  in  this  case  hap- 
pened to  be  740,  as  would  be  seen  by  referring  to  Everett,  "Units  and  Physical 
Constants."  The  product  of  a  weber  and  a  volt  was  107  ergs  per  second  (p.  138), 
while  a  horse  power  was  7.46  x  109  =  740  X  107  ergs  per  second  (p.  5J5);  hence  the 
rule  given. 


40      DYNAMO    MACHINERY   AND    ALLIED   SUBJECTS. 


SOME  POINTS  IN  ELECTRIC   LIGHTING.  • 

ARTIFICIAL  light  is  generally  produced  by  raising  some 
body  to  a  high  temperature.  If  the  temperature  of  a  body 
be  greater  than  that  of  surrounding  bodies  it  parts  with 
some  of  its  energy  in  the  form  of  radiation.  While  the 
temperature  is  low  these  radiations  are  not  of  a  kind  to 
which  the  eye  is  sensitive;  they  are  exclusively  radiations 
less  refrangible  and  of  greater  wave  length  than  red  light, 
and  may  be  called  infra  red.  As  the  temperature  is  in- 
creased the  infra  red  radiations  increase,  but  presently 
there  are  added  radiations  which  the  eye  perceives  as  red 
light.  As  the  temperature  is  further  increased,  the  red 
light  increases,  and  yellow,  green  and  blue  rays  are  succes- 
sively thrown  off  in  addition.  On  pushing  the  temperature 
to  a  still  higher  point,  radiations  of  a  wave  length  shorter 
even  than  violet  light  are  produced,  to  which  the  eye  is 
insensitive,  but  which  act  strongly  on  certain  chemical 
substances;  these  may  be  called  ultra  violet  rays.  It  is 
thus  seen  that  a  very  hot  body  in  general  throws  out  rays 
of  various  wave  lengths, — our  eyes,  it  so  happens,  being  only 
sensitive  to  certain  of  these,  viz.,  those  not  very  long  and 
not  very  short,— and  that  the  hotter  the  body  the  more  of 
every  kind  of  radiation  will  it  throw  out ;  but  the  propor- 
tion of  short  waves  to  long  waves  becomes  vastly  greater  as 
the  temperature  is  increased,  The  problem  of  the  artificial 


SOME   POINTS   IN    ELECTRIC    LIGHTING.  41 

production  of  light  with  economy  of  energy  is  the  same  as 
that  of  raising  some  body  to  such  a  temperature  that  it 
shall  give  as  large  a  proportion  as  possible  of  those  rays 
which  the  eye  happens  to  be  capable  of  feeling.  For  prac- 
tical purposes  this  temperature  is  the  highest  temperature 
we  can  produce.  Owing  to  the  high  temperature  at 
which  it  remains  solid,  and  to  its  great  emissive  power,  the 
radiant  body  used  for  artificial  illumination  is  nearly  always 
some  form  of  carbon.  In  the  electric  current  we  have  an 
agent  whereby  we  can  convert  more  energy  of  other  forms 
into  heat  in  a  small  space  than  in  any  other  way;  and 
fortunately  carbon  is  a  conductor  of  electricity  as  well  as  a 
very  refractory  substance. 

The  science  of  lighting  by  electricity  very  naturally 
divides  itself  into  two  principal  parts — the  methods  of 
production  of  electric  currents,  and  of  conversion  of  the 
energy  of  those  currents  into  heat  at  such  a  temperature 
as  to  be  given  oif  in  radiations  to  which  our  eyes  are  sensi- 
ble. There  are  other  subordinate  branches  of  the  subject, 
such  as  the  consideration  of  the  conductors  through  which 
the  electric  energy  is  transmitted,  and  the  measurement  of 
the  quantity  of  electricity  passing  and  its  potential  or  elec- 
tric pressure.  Although  I  shall  have  a  word  or  two  to  say  on 
the  other  branches  of  the  subject,  I  propose  to  occupy  most 
of  the  time  at  my  disposal  this  evening  with  certain  points 
concerning  the  conversion  of  mechanical  energy  into  elec- 
trical energy.  We  know  nothing  as  to  what  electricity  is, 
and  its  appeals  to  our  senses  are  in  general  less  direct  than 
those  of  the  mechanical  phenomena  of  matter.  The  laws, 
however,  which  we  know  to  connect  together  those  phe- 
nomena which  we  call  electrical  are  essentially  mechanical 


42      DYNAMO   MACHINEKY    AND   ALLIED   SUBJECTS. 


in  form,  are  closely  correlated  with  mechanical  laws,  and 
may  be  most  aptly  illustrated  by  mechanical  analogues. 
For  example,  the  terms  "  potential/'  "  current  "  and  "  re- 
sistance/' with  which  we  are  becoming  familiar  in  electric- 
ity, have  close  analogues  respectively  in  "head,"  "rate  of 
flow"  and  "coefficient  of  friction"  in  the  hydraulic  trans- 
mission of  power.  Exactly  as  in  hydraulics  head  multi- 
plied by  velocity  of  flow  is  power  measured  in  foot-pounds 
per  second  or  in  horse  power,  so  potential  multiplied  by 
current  is  power  and  is  measurable  in  the  same  units.  The 
horse  power  not  being  a  convenient  elec- 
trical unit,  Dr.  Siemens  has  suggested  that 
the  electrical  unit  of  power  or  volt-ampere 
should  be  called  a  watt :  746  watts  are  equal 
to  one  horse  power.  Again,  just  as  water 
flowing  in  a  pipe  has  inertia  and  requires  an 
expenditure  of  work  to  set  it  in  motion,  and 
is  capable  of  producing  disruptive  effects  if 
its  motion  is  too  suddenly  arrested, — as,  for 
example,  when  a  plug  tap  is  suddenly  closed 
in  a  pipe  through  which  water  is  flowing 
rapidly, — so  a  current  of  electricity  in  a  wire 
has  inertia;  to  set  it  moving  electromotive 
force  must  work  for  a  finite  time,  and  if  we 
r  in  iiiiiiim  attempt  to  arrest  it  suddenly  by  breaking  the 

1  circuit,  the  electricity  forces  its  way  across 

the  interval  as   a   spark.     Corresponding  to 
mass  and  moments  of  inertia  in  mechanics 
FIG.  12.         we  have    in    electricity    coefficients   of   self 
induction.      We    will    now    show    that    an 
electric  circuit  behaves  as  though  it  had  inertia.     The  ap- 


SOME   POINTS   IN    ELECTRIC    LIGHTING. 


43 


paratus  we  shall  use  is  shown  diagrammatically  in  Fig.  12. 
A  current  from  a  Sellon  battery  A  circulates  round  an 
electromagnet  Z?/  it  can  be  made  and  broken  at  pleasure 
at  C.  Connected  to  the  two  extremities  of  the  wire  on  the 


FIG.  13. 

magnet  is  a  small  incandescent  lamp  D,  lent  to  me  by  Mr. 
Crompton,  of  many  times  the  resistance  of  the  coil.  On 
breaking  the  circuit,  the  current  in  the  coil,  in  virtue  of  its 
momentum,  forces  its  way  through  the  lamp,  and  renders 
it  momentarily  incandescent,  although  all  connection  with 
the  battery,  which  in  any  case  would  be  too  feeble  to  send 
sufficient  current  through  the  lamp,  has  ceased.  Let  us. 
try  the  experiment,  make  contact,  break  contact.  You 


44      DYNAMO   MACHINERY    AND   ALLIED    SUBJECTS. 

observe  the  lamp  lights  up.  Compare  with  the  diagram 
(Fig.  13)  ^of  the  hydraulic  analogue,  the  hydraulic  ram. 
There  a  current  of  water  suddenly  arrested  forces  a  way 
for  a  portion  of  its  quantity  to  a  greater  height  than  that 
from  which  it  fell.  A  B  corresponds  to  the  electromag- 
net, the  valve  C  to  the  contact  breaker,  and  D  E  to  the 
lamp.  There  is,  however,  this  difference  between  the  in- 
ertia of  water  in  a  pipe  and  the  inertia  of  an  electric  cur- 
rent :  the  inertia  of  the  water  is  confined  to  the  water, 
whereas  the  inertia  of  the  electric  current  resides  in  the 
surrounding  medium.  Hence  arise  the  phenomena  of  in- 
duction of  currents  upon  currents,  and  of  magnets  upon 
moving  conductors — phenomena  which  have  no  immediate 
analogues  in  hydraulics.  There  is  thus  little  difficulty  to 
any  one  accustomed  to  the  laws  of  rational  mechanics  in 
adapting  the  expression  of  those  laws  to  fit  electrical 
phenomena;  indeed  we  may  go  so  far  as  to  say  that  the 
part  of  electrical  science  with  which  we  have  to  deal  this 
evening  is  essentially  a  branch  of  mechanics,  and  as  such 
I  shall  endeavor  to  treat  it. 

This  is  neither  the  time  nor  the  place  for  setting  forth 
the  fundamental  laws  of  electricity,  but  I  cannot  forbear 
from  showing  you  a  mechanical  illustration,  or  set  of 
mechanical  illustrations,  of  the  laws  of  electrical  induction, 
first  discovered  by  Faraday.  I  have  here  a  model,  Fig.  14, 
which  was  made  to  the  instructions  of  the  late  Professor 
Clerk  Maxwell,  to  illustrate  the  laws  of  induction.  It 
consists  of  a  pulley  P,  which  I  now  turn  with  my  hand, 
and  which  represents  one  electric  circuit,  its  motion  the 
current  therein.  Here  is  a  second  pulley,  S,  representing 
a  second  electric  circuit.  These  two  pulleys  are  geared 


SOME  POINTS  IN  ELECTBIC  LIGHTING.  45 


FIGK  14, 


46    DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

together  by  a  simple  differential  train,  such  as  is  some- 
times used  for  a  dynamometer.  The  intermediate  wheel 
of  the  train,  however,  is  attached  to  a  balanced  flywheel, 
the  moment  of  inertia  of  which  can  be  varied  by  moving 
inwards  or  outwards  these  four  brass  weights.  The  resist- 
ances of  the  two  electric  circuits  are  represented  by  the 
friction  on  the  pulleys  of  two  strings,  the  tension  of  which 
can  be  varied  by  tightening  these  elastic  bands.  The  dif- 
ferential train,  with  its  flywheel,  represents  the  medium, 
whatever  it  may  be,  between  the  two  electric  conductors. 
The  mechanical  properties  of  this  me  del  are  of  course 
obvious  enough.  Although  the  mathematical  equations 
which  represent  the  relation  between  one  electric  conduct- 
or and  another  in  its  neighborhood  are  the  same  in  form 
as  the  mathematical  equations  which  represent  the  mechan- 
ical connection  between  these  two  pulleys,  it  must  not  be  as- 
sumed that  the  magnetic  mechanism  is  completely  repre- 
sented by  the  model.  We  shall  now  see  how  the  model 
illustrates  the  action  of  one  electric  circuit  upon  another. 
You  know  that  Faraday  discovered  that  if  you  have  two 
closed  conductors  arranged  near  to  and  parallel  to  each 
other,  and  if  you  cause  a  current  of  electricity  to  begin  to 
flow  in  the  first,  there  will  arise  a  temporary  current  in  the 
opposite  direction  in  the  second.  This  pulley,  marked  P 
on  the  diagram,  represents  the  primary  circuit,  and  the 
pulley  marked  8  on  the  diagram  the  secondary  circuit. 
We  cause  a  current  to  begin  to  flow  in  the  primary,  or  turn 
the  pulley  P;  an  opposite  current  is  induced  in  the  sec- 
ondary circuit,  or  the  pulley  8  turns  in  the  opposite 
direction  to  that  in  which  we  began  to  move  the  pulley  P. 
The  effect  is  only  temporary;  resistance  speedily  stops  the 


SOME  POINTS   IN   ELECTRIC   LIGHTING.  47 

current  in  the  secondary  circuit,  or,  in  the  mechanical 
model,  friction  the  rotation  of  the  pulley  S.  I  now  grad- 
ually stop  the  motion  of  P;  the  pulley  S  moves  in  the 
direction  in  which  P  was  previously  moving,  just  as  Far- 
aday found  that  the  cessation  of  the  primary  current  in- 
duced in  the  secondary  circuit  a  current  in  the  same  direc- 
tion as  that  which  had  existed  in  the  primary.  If  there 
were  a  large  number  of  convolutions  or  coils  in  the  second- 
ary circuit,  but  that  circuit  were  not  completed,  but  had 
an  air  space  interrupting  its  continuity,  an  experiment 
with  the  well  known  Kuhmkorff  coil  would  show  you  that 
when  the  current  was  suddenly  made  to  cease  to  flow  in 
the  primary  circuit,  so  great  'an  electromotive  force  would 
be  exerted  in  the  secondary  circuit  that  the  electricity 
would  leap  across  the  space  as  a  spark.  I  will  now  show 
you  what  corresponds  to  a  spark  with  this  mechanical 
model.  The  secondary  pulley  S  shall  be  held  by  passing  a 
thread  several  times  round  it.  I  gradually  produce  the 
current  in  the  primary  circuit.  I  will  now  suddenly  stop 
this  primary  current:  you  observe  that  the  electromotive 
force  is  sufficient  to  break  the  thread.  The  inductive 
effects  of  one  electric  circuit  upon  another  depend  not 
alone  on  the  dimensions  and  form  of  the  two  circuits,  but 
on  the  nature  of  the  material  between  them.  For  example, 
if  we  had  two  parallel  circular  coils,  their  inductive  effects 
would  be  very  considerably  enhanced  by  introducing  a  bar 
of  iron  in  their  common  axis.  We  can  imitate  this  effect 
by  moving  outwards  or  inwards  these  brass  weights.  In 
the  experiment  I  have  shown  you  the  weights  have  been 
some  distance  from  the  axis  in  order  to  obtain  considerable 
effect,  just  as  in  the  Ruhmkorff  coil  an  iron  core  is  intro- 


48      DYNAMO   MACHINERY    A.ND  ALLIED   SUBJECTS. 


duced  within  the  primary  circuit.  I  will  now  do  what  is 
equivalent  to  removing  the  core :  I  will  bring  the  weights 
nearer  to  the  axis,  so  that  my  flywheel  shall  have  less 
moment  of  inertia.  You  observe  that  the  inductive  effects 
are  very  much  less  marked  than  they  were  before.  With 
the  same  electromagnet  which  we  used  before,  but  differ- 
ently arranged,  we  will  show  what  we  have 
just  illustrated — the  induction  of  one 
circuit  on  another.  Referring  to  Fig.  15, 
coil  A  B  corresponds  to  wheel  P ;  C  D  to 
wheel  8,  and  the  iron  core  to  the  fly- 
wheel and  differential  gear.  The  resist- 
ance of  a  lamp  takes  the  place  of  the 
friction  of  the  string  on  S.  As  we  make 
and  break  the  circuit  you  see  the  effect 
of  the  induced  current  in  rendering  the 
lamp  incandescent.  So  far  I  have  been 
illustrating  the  phenomena  of  the  induc- 
tion of  one  current  upon  another.  I  will 
now  show  on  the  model  that  a  current 
in  a  single  electric  circuit  has  momen- 
tum. The  secondary  wheel  shall  be 
firmly  held ;  it  shall  have  no  conductivity 
at  all — that  is,  its  electrical  effect  shall 
be  as  though  it  were  not  there.  I  now 
cause  a  current  to  begin  to  flow  in  the  primary  circuit, 
and  it  is  obvious  enough  that  a  certain  amount  of  work 
must  be  done  to  bring  it  up  to  a  certain  speed.  The  an- 
gular velocity  of  the  flywheel  is  half  that  of  the  pulley 
representing  the  primary  circuit.  Now  suppose  that  the 
two  pulleys  were  connected  together  in  such  a  way  that 


FIG.  15. 


SOME   POINTS   IN   ELECTRIC   LIGHTING. 


49 


they  must  have  the  same  angular  velocity  in  the  same 
direction.  This  represents  the  coil  having  twice  as  many 
convolutions  as  it  had  before.  A  little 
consideration  will  show  that  I  must  do 
four  times  as  much  work  to  give  the 
primary  pulley  the  same  velocity  that 
it  attained  before;  that  is  to  say,  that 
the  coefficient  of  self  induction  of  a  coil 
of  wire  is  proportional  to  the  square  of 
the  number  of  convolutions.  Again, 
suppose  that  these  two  wheels  were  so 
geared  together  that  they  must  always 
have  equal  and  opposite  velocities,  you 
can  see  that  a  very  small  amount  of 
work  must  be  done  in  order  to  give  the 
primary  wheel  the  velocity  which  we 
gave  to  it  before.  Such  an  arrangement 
of  the  model  represents  an  electric  cir- 
cuit, the  coefficient  of  induction  of  which 
is  exceedingly  small,  such  as  the  coils 
that  are  wound  for  standard  resistances; 
the  wire  is  there  wound  double,  and 
the  current  returns  upon  itself,  as  shown 
in  Fig.  16. 

In  the  widest  sense,  the  dynamo-electric  machine  may 
be  defined  as  an  apparatus  for  converting  mechanical 
energy  into  the  energy  of  electrostatic  charge,  or  mechan- 
ical power  into  its  equivalent  electric  current  through  a 
conductor.  Under  this  definition  would  be  included  the 
electrophorus  and  all  f rictional  machines ;  but  the  term  is 
used,  in  a  more  restricted  sense,  for  those  machines  which 


FIG.  16. 


50     DYtfAMO   MACHINEHY  AND   ALLIED   SUBJECTS. 

produce  electric  currents  by  the  motion  of  conductors  in  a 
magnetic  field,  or  by  the  motion  of  a  magnetic  field  in  the 
neighborhood  of  a  conductor.  The  laws  on  which  the 
action  of  such  machines  is  based  have  been  the  subject  of 
a  series  of  discoveries.  Oersted  discovered  that  an  electric 
current  in  a  conductor  exerted  force  upon  a  magnet; 
Ampere  that  two  conductors  conveying  currents  generally 
exerted  a  mechanical  force  upon  each  other.  Faraday  dis- 
covered— what  Helmholtz  and  Thomson  subsequently 
proved  to  be  the  necessary  consequence  of  the  mechanical 
reactions  between  conductors  conveying  currents  and  mag- 
nets— that  if  a  closed  conductor  move  in  a  magnetic  field, 
there  will  be  a  current  induced  in  that  conductor  in  one 
direction  if  the  number  of  lines  of  magnetic  force  passing 
through  the  conductor  was  increased  by  the  movement;  in 
the  other  direction  if  diminished.  Now  all  dynamo-electric 
machines  are  based  upon  Faraday's  discovery.  Not  only 
so ;  but  however  elaborate  we  may  wish  to  make  the  analysis 
of  the  action  of  a  dynamo  machine,  Faraday's  way  of  pre- 
senting the  phenomena  of  electromagnetism  to  the  mind 
is  in  general  our  best  point  of  departure.  The  dynamo 
machine,  then,  essentially  consists  of  a  conductor  made  to 
move  in  a  magnetic  field.  This  conductor,  with  the  exter- 
nal circuit,  forms  a  closed  circuit  in  which  electric  currents 
are  induced  as  the  number  of  lines  of  magnetic  force  pass- 
ing through  the  closed  circuit  varies.  Since,  then,  if  the 
current  in  a  closed  circuit  be  in  one  direction  when  the 
number  of  lines  of  force  is  increasing,  and  in  the  opposite 
direction  when  they  are  diminishing,  it  is  clear  that  the 
current  in  each  part  of  such  circuit  which  passes  through 
the  magnetic  field  must  be  alternating  in  direction,  unless, 


SOME  POINTS  IN   ELECTRIC   LIGHTING.  51 

indeed,  the  circuit  be  such  that  it  is  continually  cutting 
more  and  more  lines  of  force,  always  in  the  same  direction. 
Since  the  current  in  the  wire  of  the  machine  is  alternating, 
so  also  must  be  the  current  outside  the  machine,  unless 
something  in  the  nature  of  a  commutator  be  employed  to 
reverse  the  connections  of  the  internal  wires  in  which  the 
current  is  induced,  and  of  the  external  circuit.  We  have, 
then,  broadly,  two  classes  of  dynamo-electric  machines — 
the  simplest,  the  alternating  current  machine,  where  no 
commutator  is  used;  and  the  continuous  current  machine, 
in  which  a  commutator  is  used  to  change  the  connection 
of  the  external  circuit  just  at  the  moment  when  the  direc- 
tion of  the  current  would  change.  The  mathematical 
theory  of  the  alternate  current  machine  is  comparatively 
simple.  To  fix  ideas,  I  will  ask  you  to  think  of  the  alter- 
nate current  Siemens  machine,  which  Dr.  Siemens  exhibited 
here  three  weeks  ago.  We  have  there  a  series  of  magnetic 
fields  of  alternate  polarity,  and  through  these  fields  we 
have  coils  of  wire  moving;  these  coils  constitute  what  is 
called  the  armature;  in  them  are  induced  the  currents 
which  give  a  useful  effect  outside  the  machine.  Now  I 
am  not  going  to  trouble  you  to  go  through  the  mathematical 
equations,  simple  though  they  are,  by  which  the  following 
formulae  are  obtained: — 


n  t  /T , 

r  (I.) 


2  n  A        2  Ttt 
E=   --cos-  (II.) 


52    DYNAMO  MACHINERY  AND  ALLIED 

£=          (in.) 

(IV.) 


(VL) 


T  represents  the  periodic  time  of  the  machine ;  that  is,  in 
the  case  of  a  Siemens  machine  having  eight  magnets  on 
each  side  of  the  armature,  T  represents  the  time  of  one- 
fourth  of  a  revolution.  /  represents  the  number  of  lines 
of  force  embraced  by  the  coils  of  the  armature  at  the  time 
t.  I  must  be  a  periodic  function  of  /,  in  the  simplest  form 
represented  by  Equation  I.  Equation  II.  gives  E  the  elec- 
tromotive force  acting  at  time  /  upon  the  circuit.  Having 
given  the  electromotive  force  acting  at  any  time,  it  would 
appear  at  first  sight  that  we  had  nothing  to  do  but  to 
divide  that  electromotive  force  by  the  resistance  R  of  the 
whole  circuit,  to  obtain  the  current  flowing  at  that  time. 
But  if  we  were  to  do  so  we  should  be  landed  in  error,  for 
the  conducting  circuit  has  other  properties  besides  resist- 
ance. I  pointed  out  to  you  that  it  had  a  property  of  mo- 
mentum represented  by  its  coefficient  of  self  induction, 


LIGHTING. 


53 


with 

it  |iijii  at  important  a  part  as 
DI.  gives  die 
wiD  obeerre  that  it 

Hlesstkanitwoaldbeif 

byfte 

br  Foonia  IV. 
of  deefetical  work 


--;,-_:_,  ::::_.:     :   -^ 


54      DYNAMO  "MACHINERY   AND   ALLIED   SUBJECTS. 

In  some  cases  this  phenomenon  is  so  marked  that  the 
machine  actually  takes  more  to  drive  it,  when  the  machine 
is  on  open  circuit,  than  when  it  is  short  circuited.  The  ex- 
planation is  that  on  open  circuit  currents  are  induced  in 
the  iron  cores,  but  that  when  the  copper  coils  are  closed 
the  current  in  them  diminishes  by  induction  the  current  in 
the  iron.  The  effect  of  currents  in  the  iron  cores  is  not 
alone  to  waste  eiwrgy  and  heat  the  machine;  but  for  a 
given  intensity  of  field  and  speed  of  revolution  the  exter- 
nal current  produced  is  diminished.  The  cure  of  the  evil 
is  to  subdivide  the  moving  iron  as  much  as  possible,  in  di- 
rections perpendicular  to  those  in  which  the  current  tends 
to  circulate. 

There  remains  one  point  of  great  practical  interest  in 
connection  with  alternate  current  machines:  How  will 
they  behave  when  two  or  more  are  coupled  together  to  aid 
each  other  in  doing  the  same  work  ?  With  galvanic  bat- 
teries we  know  very  well  how  to  couple  them,  either  in 
parallel  circuit  or  in  series,  so  that  they  shall  aid,  and  not 
oppose,  the  effects  of  each  other;  but  with  alternate  cur- 
rent machines,  independently  driven,  it  is  not  quite  obvi- 
ous what  the  result  will  be,  for  the  polarity  of  each 
machine  is  constantly  changing.  Will  two  machines, 
coupled  together,  run  independently  of  each  other,  or  will 
one  control  the  movement  of  the  other  in  such  wise 
that  they  settle  down  to  conspire  to  produce  the  same 
effect,  or  will  it  be  into  mutual  opposition  ?  It  is  obvious 
that  a  great  deal  turns  upon  the  answer  to  this  question, 
for  in  the  general  distribution  of  electric  light  it  will  be 
desirable  to  be  able  to  supply  the  system  of  conductors 
from  which  the  consumers  draw  by  separate  machines, 


SOME   POINTS   IN   ELECTRIC   LIGHTING.  OO 

which  can  be  thrown  in  and  out  at  pleasure.  Now  I  know 
it  is  a  common  impression  that  alternate  current  machines 
cannot  be  worked  together,  and  that  it  is  almost  a  necessity 
to  have  one  enormous  machine  to  supply  all  the  consumers 
drawing  from  one  system  of  conductors.  Let  us  see  how 
the  matter  stands.  Consider  two  machines  independently 
driven,  so  as  to  have  approximately  the  same  periodic  time 


FIG.  17. 


and  the  same  electromotive  force.  If  these  two  machines 
are  to  be  worked  together,  they  may  be  connected  in  one 
of  two  ways :  they  may  be  in  parallel  circuit  with  regard  to 
the  external  conductor,  as  shown  by  the  full  line  in  Fig.  1 7, 
that  is,  their  currents  may  be  added  algebraically  and  sent 
to  the  external  circuit,  or  they  may  be  coupled  in  series,  as 
shown  by  the  dotted  line,  that  is,  the  whole  current  may 
pass  successively  through  the  two  machines,  and  the 
electromotive  force  of  the  two  machines  may  be  added  ? 


56      DYNAMO  MACHINERY  AND  ALLIED   SUBJECTS. 

instead  of  their  currents.  The  latter  case  is  simpler.  Let 
us  consider  it  first.  I  am  going  to  show  that  if  you  couple 
two  such  alternate  current  machines  in  series  they  will  so 
control  each  other's  phase  as  to  nullify  each  other,  and 
that  you  will  get  no  effect  from  them ;  and,  as  a  corollary 
from  that,  I  am  going  to  show  that  if  you  couple  them  in 
parallel  circuit  they  will  work  perfectly  well  together,  and 
the  currents  they  produce  will  be  added;  in  fact,  that  you 
cannot  drive  alternate  current  machines  tandem,  but  that 
you  may  drive  them  as  a  pair,  or,  indeed,  any  number 
abreast.  In  diagram,  Fig.  18,  the  horizontal  line  of  ab- 


1JIL11IV 


scissae  represents  the  time  advancing  from  left  to  right; 
the  full  curves  represent  the  electromotive  forces  of  the 
two  machines  not  supposed  to  be  in  the  same  phase.  We 
want  to  see  whether  they  will  tend  to  get  into  the  same 
phase  or  to  get  into  opposite  phases.  Now,  if  the  machines 
are  coupled  in  series,  the  resultant  electromotive  force 
on  the  circuit  will  be  the  sum  of  the  electromotive 
forces  of  the  two  machines.  This  resultant  electromotive 
force  is  represented  by  the  broken  curve  III.  By  what  we 
have  already  seen  in  Formula  IV.,  the  phase  of  the  cur- 


SOME   POINTS   IN   ELECTRIC   LIGHTING.  57 

reut  must  lag  behind  the  phase  of  the  electromotive  force, 

as  is  shown  in  the  diagram  by  curve  7F,  thus . . 

.     Now  the  work  done  in  any  machine  is  represented 

by  the  sum  of  the  products  of  the  currents  and  of  the 
electromotive  forces,  and  it  is  clear  that,  as  the  phase  of 
the  current  is  more  near  to  the  phase  of  the  lagging 
machine  //  than  to  that  of  the  leading  machine  /,  the  lag- 
ging machine  must  do  more  work  in  producing  electricity 
than  the  leading  machine;  consequently  its  velocity 
will  be  retarded,  and  its  retardation  will  go  on  until  the 
two  machines  settle  down  into  exactly  opposite  phases, 
when  no  current  will  pass.  The  moral,  therefore,  is,  do 
not  attempt  to  couple  two  independently  driven  alternate 
current  machines  in  series.  Now  for  the  corollary:  A,  B, 
Fig.  17,  represent  the  two  terminals  of  an  alternate  cur- 
rent machine;  «,  b  the  two  terminals  of  another  machine 
independently  driven.  A  and  a  are  connected  together, 
and  B  and  b.  So  regarded,  the  two  machines  are  in 
series,  and  we  have  just  proved  that  they  will  exactly 
oppose  each  other's  effects,  that  is,  when  A  is  positive,  a 
will  be  positive  also;  when  A  is  negative,  a  is  also  nega- 
tive. Now,  connecting  A  and  a  through  the  compara- 
tively high  resistance  of  the  external  circuit  with  B  and  b,  the 
current  passing  through  that  circuit  will  not  much  disturb, 
if  at  all,  the  relations  of  the  two  machines.  Hence,  when 
A  is  positive,  a  will  be  positive,  and  when  A  is  negative,  a 
will  be  negative  also;  precisely  the  condition  required  that 
the  two  machines  may  work  together  to  send  a  current  into 
the  external  circuit.  You  may,  therefore,  with  confi- 
dence, attempt  to  run  alternate  current  machines  in 
parallel  circuit  for  the  purpose  of  producing  any  external 


58      DYNAMO   MACHINERY   AND    ALLIED   SUBJECTS. 

effect.  I  might  easily  show  that  the  same  applies  to  a 
larger  number;  hence  there  ic  no  more  difficulty  in  feed- 
ing a  system  of  conductors  from  a  number  of  alternate  cur- 
rent machines  than  there  is  in  feeding  it  from  a  number 
of  continuous  current  machines.  A  little  care  is  only  re- 
quired that  the  machine  shall  be  thrown  in  when  it  has 
attained  something  like  its  proper  velocity.  A  further 
corollary  is  that  alternate  currents  with  alternate  current 
machines  as  motors  may  theoretically  be  used  for  the  trans- 
mission of  power.* 

It  is  easy  to  see  that,  by  introducing  a  commutator  re- 
volving with  the  armature,  in  an  alternate  current  machine, 
and  so  arranged  as  to  reverse  the  connection  between  the 
armature  and  the  external  circuit  just  at  the  time  when 
the  current  would  reverse,  it  is  possible  to  obtain  a  cur- 
rent constant  always  in  direction;  but  such  a  current 
would  be  far  from  constant  in  intensity,  and  would  cer- 
tainly not  accomplish  all  the  results  that  are  obtained  in 
modern  continuous  current  machines.  This  irregularity 
may,  however,  be  reduced  to  any  extent  by  multiplying 
the  wires  of  the  armature,  giving  eacli  its  own  connection 
to  the  outer  circuit,  and  so  placing  them  that  the  electro- 
motive force  attains  a  maximum  successively  in  the  several 
coils.  A  practically  uniform  electric  current  was  first  com- 
mercially produced  with  the  ring  armature  of  Pacinotti, 
as  perfected  by  Gramme.  The  Gramme  machine  is  repre- 
sented diagram matically  in  Fig.  19.  The  armature  consists 

*Of  course  in  applying  these  conclusions  it  is  necessary  to  remember  that 
the  machines  only  tend  to  control  each  other,  and  that  the  control  of  the 
motive  power  may  be  predominant,  and  compel  th«  two  or  more  machines  to 
run  at  different  speeds. 


SOME   POINTS   IN    ELECTRIC   LIGHTING. 


59 


of  an  anchor  ring  of  iron  wire,  the  strands  more  or  less 
insulated  from  each  other.  Round  this  anchor  ring  is 
wound  a  continuous  endless  coil 
of  copper  wire;  the  armature 
moves  in  a  magnetic  field,  pro- 
duced by  permanent  or  electro- 
magnets with  diametrically  oppo- 
site poles,  marked  N  and  S.  The 
lines  of  magnetic  force  may  be 
regarded  as  passing  into  the  ring 
from  N,  dividing,  passing  round 
the  ring  and  across  to  S.  Thus 
the  coils  of  wire,  both  near  to  N 
and  near  to  S,  are  cutting  through 
a  very  strong  magnetic  field;  con- 
sequently there  will  be  an  intense 
inductive  action.  The  inductive 
action  of  the  coils  near  JV  being 
equal  and  opposite  to  the  induc- 
tive action  of  the  coils  near  S, 
it  results  that  there  will  be  strong 
positive  and  negative  electric  po- 
tential at  the  extremities  of  a 
diameter  perpendicular  to  the  line 
NS.  The  electromotive  force  pro- 
duced is  made  use  of  to  produce  a 

current  external  to  the  machine;  thus  the  endless  coil  of 
the  armature  is  divided  into  any  number  of  sections,  in  the 
diagram  into  six  for  convenience,  usually  into  sixty  or 
eighty,  and  the  junction  of  each  pair  of  sections  is  con- 
nected by  a  wire  to  a  plate  of  the  commutator  fixed  upon 


FIG.  19. 


60      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

the  shaft  which  carries  the  armature;  collecting  brushes 
make  contact  with  the  commutator,  as  shown  in  the 
diagram.  If  the  external  resistance  were  enormously 
high,  so  that  very  little  current,  or  none  at  all,  passed 
through  the  armature,  the  greatest  difference  of  potential 
between  the  two  brushes  would  be  found  when  they  made 
contact  at  points  at  right  angles  to  the  line  between  the 
magnets;  but  when  a  current  passes  in  the  armature,  this 
current  causes  a  disturbing  effect  upon  the  magnetic  field. 
Every  time  the  contact  of  the  brushes  changes  from  one 
contact  plate  to  the  next,  the  current  in  a  section  of  the 
copper  coil  is  reversed,  and  this  reversal  has  an  inductive 
effect  upon  all  the  other  coils  of  the  armature.  You  may 
take  it  from  me  that  the  net  result  on  any  one  coil  is  approxi- 
mately the  same  as  if  that  coil  alone  were  moved,  and  all 
the  other  coils  were  fixed,  and  there  were  no  reversals  of 
current  in  them.  Now  you  can  easily  see  that  the  mag- 
netic effect  of  the  current  circulating  in  the  coils  of 
the  armature  will  be  to  produce  a  north  pole  at  n 
and  a  south  pole  at  s.  This  will  displace  the  magnetic 
field  in  the  direction  of  rotation.  If,  then,  we  were  to 
keep  the  contact  points  the  same  as  when  no  current  was 
passing,  we  should  short  circuit  the  sections  of  the  arma- 
ture at  a  time  when  they  were  cutting  through  the  lines 
of  magnetic  force,  with  a  result  that  there  would  be  vigor- 
ous sparks  between  the  collecting  brushes  and  the  com- 
mutator. To  avoid  this,  the  brushes  must  follow  the 
magnetic  field,  and  also  be  displaced  in  the  direction  of 
rotation,  this  displacement  being  greater  as  the  current  in 
the  armature  is  greater  in  proportion  to  the  magnetic  field. 
The  net  effect  of  this  disturbing  effect  of  the  current  in. 


SOME  POINTS   IN    ELECTRIC   LIGHTING.  61 

the  armature  reacting  upon  itself  is,  then,  to  displace  the 
neutral  points  upon  the  commutator,  and  consequently 
somewhat  to  diminish  the  effective  electromotive  force. 
It  is  best  to  adjust  the  brushes  to  make  contact  at  a  point 
such  that,  with  the  current  then  passing,  flashing  is  re- 
duced to  a  minimum;  but  this  point  does  not  necessarily 
coincide  with  the  point  which  gives  maximum  difference 
of  potential.  The  magnetic  field'  in  the  Gramme  and 
other  continuous  dynamo-electric  machines  may  be  pro- 
duced in  several  ways.  Permanent  magnets  of  steel  may  be 
used,  as  in  some  of  the  smaller  machines  now  made,  and  in 
all  the  earlier  machines;  these  are  frequently  called  mag- 
neto machines.  Electromagnets  excited  by  a  current 
from  a  small  dynamo-electric  machine  were  introduced 
by  Wilde;  these  may  be  described  shortly  as  dynamos 
with  separate  exciters.  The  plan  of  using  the  whole 
current  from  the  armature  of  the  machine  itself,  for 
exciting  the  magnets,  was  proposed  almost  simultaneously 
by  Siemens,  Wheatstone,  and  S.  A.  Varley.  A  dynamo 
so  excited  is  now  called  a  series  dynamo.  Another  method 
is  to  divide  the  current  from  the  armature,  sending  the 
greater  part  into  the  external  circuit,  and  a  smaller  por- 
tion through  the  electromagnet,  which  is  then  of  very 
much  higher  resistance.  Such  an  arrangement  is  called  a 
shunt  dynamo.  A  combination  of  the  last  two  methods 
has  been  recently  introduced,  for  the  purpose  of  main- 
taining constant  potential.  The  magnet  is  partly  ex- 
cited by  a  circuit  of  high  resistance,  a  shunt  to  the 
external  circuit,  and  partly  by  coils  conveying  the 
whole  current  from  the  armature.  All  but  the  first 
two  arrangements  named  depend  on  residual  magnetism 


62      DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

to  initiate  the  current,  and  below  a  certain  speed  of  rotation 
give  no  practically  useful  electromotive  force.  A  dynamo 
machine  is,  of  course,  not  a  perfect  instrument  for  converting 
mechanical  energy  into  the  energy  of  electric  current.  Cer- 
tain losses  inevitably  occur.  There  is,  of  course,  the  loss 
due  to  friction  of  bearings,  and  of  the  collecting  brushes 
upon  the  commutator;  there  is  also  the  loss  due  to  the 
production  of  electric  currents  in  the  iron  of  the  machine. 
When  these  are  accounted  for,  we  have  the  actual  electrical 
effect  of  the  machine  in  the  conducting  wire;  but  all  of 
this  is  not  available  for  external  work.  The  current  has  to 
circulate  through  the  armature,  which  inevitably  has  elec- 
trical resistance;  electrical  energy  must,  therefore,  be  con- 
verted into  heat  in  the  armature  of  the  machine.  Energy 
must  also  be  expended  in  the  wire  of  the  electromagnet 
which  produces  the  field,  for  the  resistance  of  this  also  cannot 
be  reduced  beyond  a  certain  limit.  The  loss  by  the  resistance 
of  the  wires  of  the  armature  and  of  the  magnets  greatly 
depends  on  the  dimensions  of  the  machine.  About  this 
I  shall  have  to  say  a  word  or  two  presently.  To  know  the 
properties  of  any  machine  thoroughly,  it  is  not  enough  to 
know  its  efficiency  and  the  amount  of  work  it  is  capable 
of  doing;  wo  need  to  know  what  it  will  do  under  all  cir- 
cumstances of  varying  resistance  or  varying  electromotive 
force.  We  must  know,  under  any  given  conditions,  what 
will  be  the  electromotive  force  of  the  armature.  Now  this 
electromotive  force  depends  on  the  intensity  of  the  mag- 
netic field,  and  the  intensity  of  the  magnetic  field  depends 
on  the  current  passing  round  the  electromagnet  and  the 
current  in  the  armature.  The  current,  then,  in  the  machine 
is  the  proper  independent  variable  in  terms  of  which  to 


SOME  POINTS  IN   ELECTRIC   LIGHTING.  63 

express  the  electromotive  force.  The  simplest  case  is  that 
of  the  series  dynamo,  in  which  the  current  in  the  electro- 
magnet and  in  the  armature  is  the  same,  for  then  we  have 
only  one  independent  variable.  The  relation  between  the 
electromotive  force  and  current  is  represented  by  such  a 
curve  as  is  shown  in  the  diagram,  Fig.  20.  The  abscissae, 


Fio.  20. 


measured  along  0  X,  represent  the  current,  and  the  ordi- 
nates  represent  the*  electromotive  force  in  the  armature. 
When  four  years  ago  I  first  used  this  curve,  for  the  pur- 
pose of  expressing  the  results  of  my  experiments  on  the 
Siemens  dynamo  machine,  I  pointed  out  that  it  was  capable 
of  solving  almost  any  problem  relating  to  a  particular 
machine,  and  that  it  was  also  capable  of  giving  good  indi- 
cations of  the  results  of  changes  in  the  winding  of  the 
magnets  or  of  the  armatures  of  such  machines.  Since  then 
M.  Marcel  Deprez  has  happily  named  such  curves  "  char- 


64      DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

acteristic  curves."  I  will  give  you  one  or  two  illustrations 
of  their  use.  A  complete  characteristic  of  a  series  dynamo 
does  not  terminate  at  the  origin,  but  has  a  negative  branch, 
as  shown  in  the  diagram ;  for  it  is  clear  that  by  reversing 
the  current  through  the  whole  machine  the  electromotive 
force  is  also  reversed.  Suppose  a  series  dynamo  is  used  for 
charging  an  accumulator,  and  is  driven  at  a  given  speed, 
what  current  will  pass  through  it  ?  The  problem  is  easily 
solved.  Along  0  Y,  Fig.  20,  set  off  0  E  to  represent  the 
electromotive  force  of  the  accumulator,  and  through  ^draw 
the  line  C E  B  A,  making  an  angle  with  0  X,  such  that  its 
tangent  is  equal  to  the  resistance  of  the  whole  circuit,  and 
cutting  the  characteristic  curve,  as  it  in  general  will  do,  in 
three  points,  A,  B,  and  C.  We  have,  then,  three  answers  to 
the  question.  The  current  passing  through  the  dynamo 
will  be  either  0  L,  0  M,  or  0  Nt  the  abscissae  of  the  points 
where  the  line  cuts  the  curve.  0  L  represents  the  current 
when  the  dynamo  is  actually  charging  the  accumulator. 
0  M  represents  a  current  which  could  exist  for  an  instant, 
but  which  would  be  unstable,  for  the  least  variation  would 
tend  to  increase.  0  N  is  the  current  which  passes  if  the 
current  in  the  dynamo  should  get  reversed,  as  it  is  very 
apt  to  do  when  used  for  this  purpose.  THie  next  illustration 
is  rather  outside  my  subject,  but  shows  another  method  of 
using  the  characteristic  curve.  Many  of  you  have  heard  of 
Jacobi's  law  of  maximum  effect  of  transmitting  work  by 
dynamo  machines.  It  is  this:  Supposing  that  the  two 
dynamo  machines  were  perfect  instruments  for  converting 
mechanical  energy  into  electrical  energy,  and  that  the  gen- 
erating machine  were  run  at  constant  velocity,  while  the 
receiving  machine  had  a  variable  velocity,  the  greatest 


SOME  POINTS  IN   ELECTRIC   LIGHTING. 


65 


amount  of  work  would  be  developed  in  the  receiving 
machine  when  its  electromotive  force  was  one-half  that  of 
the  generating  machine;  then  the  efficiency  would  be  one- 
half,  and  the  electrical  work  done  by  the  generating  machine 
would  be  just  one-half  of  what  it  would  be  if  the  receiving 
machine  were  forcibly  held  at  rest.  Now  this  law  is  strictly 
true  if,  and  only  if,  the  electromotive  force  of  the  generat- 
ing machine  is  independent  of  the  current  passing  through 


its  armature.  What  I  am  now  going  to  do  is  to  give  you  a 
construction  for  determining  the  maximum  work  which 
can  be  transmitted  when  the  electromotive  force  of  the 
generating  machine  depends  on  the  current  passing  through 
the  armature,  as,  indeed,  it  nearly  always  does.  Referring  to 
Fig.  21,  0  PB  is  the  characteristic  curve  of  the  generating 
machine.  Construct  a  derived  curve  thus:  at  successive 


66      DYNAMO   MACHINERY   ANt>  ALLIED 

points  P  of  the  characteristic  curve  draw  tangents  P  T; 
draw  T  N  parallel  to  0  X,  cutting  P  j^in  N;  produce  M  P 
to  L,  making  L  P  equal  P  N;  the  point  L  gives  the  derived 
curve,  which  I  want.  Now,  to  find  the  maximum  work 
which  can  be  transmitted,  draw  0  A  at  such  an  angle  with 
0  X  that  its  tangent  is  equal  to  twice  the  resistance  of  the 
whole  circuit,  cutting  the  derived  curve  in  A.  Draw  the 
ordinate  A  C,  cutting  the  characteristic  curve  in  B;  bisect 
A  (7 at  D.  The  work  expended  upon  the  generating  machine 
would  be  represented  by  the  parallelogram  0  C  B  R,  the 
work  wasted  in  resistance  by  0  CD  S,  and  the  work  de- 
veloped in  the  receiving  machine  by  the  parallelogram 
SDBR. 

When  the  dynamo  machine  is  not  a  series  dynamo,  but 
the  currents  in  the  armature  and  in  the  electromagnet, 
though  possibly  dependent  upon  each  other,  are  not  nece.s- 
sarily  equal,  the  problem  is  not  quite  so  simple.  We  have, 
then,  two  variables,  the  current  in  the  electromagnet  and 
the  current  in  the  armature;  and  the  proper  representation 
of  the  properties  of  the  machine  will  be  by  a  characteristic 
surface  such  as  that  illustrated  by  this  model,  Fig.  22.  Of 
the  three  co-ordinate  axes,  0  X  represents  the  current  in 
the  magnet,  0  Y  represents  the  current  in  the  armature, 
not  necessarily  to  the  same  scale,  and  0  Z  the  electromo- 
tive force.  By  the  aid  of  such  a  surface  as  this,  one  may 
deal  with  any  problem  relating  to  a  dynamo  machine,  no 
matter  how  its  electromagnets  and  its  armature  are  con- 
nected together.  Let  us  apply  the  model  to  find  the 
characteristic  of  a  series  dynamo.  Take  a  plane  through 
0  Zy  the  axis  of  electromotive  force,  and  making  such  an 
angle  with  the  plane  0  X,  0  Z  that  its  tangent  is  equal  to 


SOME  POINTS   IN   ELECTRIC   LIGHTING. 


67 


current  unity  on  axis  0  Y,  divided  by  current  unity  on 
axis  0  X.  This  plane  cuts  the  surface  in  a  curve.  The 
projection  of  this  curve  on  the  plane  OX,  OZ  is  the 
characteristic  curve  of  the  series  dynamo.  This  model  only 
shows  an  eighth  part  of  the  complete  surface.  If  any  of 
you  should  interest  yourselves  about  the  other  seven  parts, 
which  are  not  without  interest,  remember  that  it  is  assumed 


FIG.  22. 

that  the  brushes  always  make  contact  with  the  commu- 
tator at  the  point  of  no  flashing,  if  there  is  one.  Of  course 
in  actual  practice  one  would  not  use  the  model  of  the 
surface,  but  the  projections  of  its  sections.  While  I  am 
speaking  of  characteristic  curves  there  is  one  point  I  will 
just  take  this  opportunity  of  mentioning.  Three  years  ago 
Mr.  Shoolbred  exhibited  the  characteristic  curve  of  a 
Gramme  machine,  in  which,  after  the  current  attained  to  a 


68     DYNAMO  MACHINERY   AND  ALLIED  SUBJECTS. 

certain  amount,  the  electromotive  force  began  to  fall.  I 
then  said  that  I  thought  there  must  be  some  mistake  in  the 
experiment.  However,  subsequent  experiments  have  veri- 
fied the  fact;  and  when  one  considers  it,  it  is  not  very 
difficult  to  see  the  explanation.  It  lies  in  this :  after  the 
current  attains  to  a  certain  amount  the  iron  in  the  machines 
becomes  magnetically  nearly  saturated,  and  consequently 
an  increase  in  the  current  does  not  produce  a  correspond- 
ing increase  in  the  magnetic  field.  The  reaction,  however, 
between  the  different  sections  of  the  wire  on  the  armature 
goes  on  increasing  indefinitely,  and  its  effect  is  to  diminish 
the  electromotive  force. 

A  little  while  ago  I  said  that  the  dimensions  of  the 
machine  had  a  good  deal  to  do  with  its  efficiency.  Let  us 
see  how  the  properties  of  a  machine  depend  upon  its  dimen- 
sions. Suppose  two  machines  alike  in  every  particular  ex- 
cepting that  the  one  has  all  its  linear  dimensions  double 
those  of  the  other;  obviously  enough  all  the  surfaces  in  the 
larger  would  be  four  times  the  corresponding  surfaces  in 
the  smaller,  and  the  weights  and  volumes  of  the  larger 
would  be  eight  times  the  corresponding  weights  in  the 
smaller  machine.  The  electrical  resistances  in  the  larger 
machine  would  be  one-half  those  of  the  smaller.  The  cur- 
rent required  to  produce  a  given  intensity  of  magnetic  field 
would  be  twice  as  great  in  the  larger  machine  as  in  the 
smaller.  In  the  diagram  (Fig.  23)  are  shown  the  compara- 
tive characteristic  curves  of  the  two  machines,  when  driven 
at  the  same  speed.  You  will  observe  that  one  curve 
is  the  projection  of  the  other,  having  corresponding 
points  with  abscissae  in  the  ratio  of  one  to  two,  and  the 
ordinates  in  the  ratio  of  one  to  four.  Now  at  first  sight  it 


SOME   POINTS   IN   ELECTRIC   LIGHTING. 


69 


would  seem  as  though,  since  the  wire  on  the  magnet  and 
armature  of  the  larger  machine  has  four  times  the  section 
of  that  of  the  smaller,  four  times  the  current  could  be 
carried,  that  consequently  the  intensity  of  the  magnetic 
field  would  be  twice  as  great  and  its  area  would  be  four 
times  as  great,  and  hence  the  electromotive  force  eight 
times  as  great;  and,  since  the  current  in  the  armature  also 
is  supposed  to  be  four  times  as  great,  that  the  work  done 


by  the  larger  machine  would  be  thirty-two  times  as  much 
as  that  which  would  be  done  by  the  smaller.  Practically, 
however,  no  such  result  can  possibly  be  obtained,  for  a 
whole  series  of  reasons.  First  of  all,  the  iron  of  the  mag- 
nets becomes  saturated,  and  consequently,  instead  of  getting 
eight  times  the  electromotive  force,  we  should  only  get  four 
times  the  electromotive  force.  Secondly,  the  current  which 
we  can  carry  in  the  armature  is  limited  by  the  rate  at  which 
we  can  get  rid  of  the  heat  generated  in  the  armature.  This 
we  may  consider  as  proportional  to  its  surface;  consequently 


70      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

we  must  only  waste  four  times  as  much  energy  in  the  arma- 
ture of  the  larger  machine  as  in  the  smaller  one,  instead  of 
eight  times,  as  would  be  the  case  if  we  carried  the  current 
in  proportion  to  the  section  of  the  wire.  Again,  the  larger 
machine  cannot  run  at  so  great  an  angular  velocity  as  the 
smaller  one.  And  lastly,  since  in  the  larger  machine  the 
current  in  the  armature  is  greater  in  proportion  to  the 
saturated  magnetic  field  than  it  is  in  the  smaller  one,  the 
displacement  of  the  point  of  contact  of  the  brushes  with 
the  commutator  will  be  greater.  However,  to  cut  the 
matter,  about  which  one  might  say  a  great  deal,  short,  one 
may  say  that  the  capacity  of  similar  dynamo  machines  is 
pretty  much  proportionate  to  their  weight,  that  is,  to  the 
cube  of  their  linear  dimensions;  that  the  work  wasted  in 
producing  the  magnetic  field  will  be  directly  as  the  linear 
dimensions;  and  that  the  work  wasted  in  heating  the  wires 
of  the  armature  will  be  as  the  square  of  the  linear  dimensions. 
Now  let  us  see  how  this  would  practically  apply.  Suppose 
we  had  a  small  machine  capable  of  producing  an  electric 
current  of  4  h.  p.,  that  of  this  4  h.  p.  1  was  wasted  in  heat- 
ing the  wires  of  the  armature,  and  1  in  heating  the  wires 
of  the  magnet;  2  would  be  usefully  applied  outside.  Now 
if  we  doubled  the  linear  dimensions  we  should  have  a  ca- 
pacity of  32  h.  p.,  of  which  2  only,  if  suitably  applied,  would 
be  required  to  produce  the  magnetic  field,  and  4  would  be 
wasted  in  heating  the  wires  of  the  armature,  leaving  26  h.  p. 
available  for  useful  work  outside  the  machine — a  very  dif- 
ferent economy  from  that  of  the  smaller  machines.  But 
if  we  again  doubled  the  linear  dimensions  of  our  machine, 
we  should  by  no  means  obtain  a  similar  increase  of  effect. 
A  consideration  of  the  properties  of  similar  machines  has 


SOME   POINTS   IN   ELECTRIC   LIGHTING.  71 

another  very  important  practical  use.  As  you  all  know, 
Mr.  Froude  was  able  to  control  the  design  of  ironclad  ships 
by  experiments  upon  models  made  in  paraffin  wax.  Now 
it  is  a  very  much  easier  thing  to  predict  what  the  perform- 
ance of  a  large  dynamo  machine  will  be,  from  laboratory 
experiments  made  upon  a  model  of  a  very  small  fraction  of 
its  dimensions.  As  a  proof  of  the  practical  utility  of  such 
methods,  I  may  say  that  by  laboratory  experiments  I  have 
succeeded  in  increasing  the  capacity  of  the  Edison  machines 
without  increasing  their  cost,  and  with  a  small  increase  of 
their  percentage  of  efficiency,  remarkably  high  as  that 
efficiency  already  was. 

I  might  occupy  your  time  with  considerations  as  to  the 
proper  proportion  of  conductors,  and  explain  Sir  W. 
Thomson's  law  that  the  most  economical  size  of  a  copper 
conductor  is  such  that  the  annual  charge  for  interest  and 
depreciation  of  the  copper  of  which  it  is  made  shall  be 
equal  to  the  cost  of  producing  the  power  which  is  wasted 
by  its  resistance.  But  the  remaining  time  will,  perhaps,  be 
best  spent  in  considering  the  production  of  light  from  the 
energy  of  electric  currents.  You  all  know  that  this  is  done 
commercially  in  two  ways— by  the  electric  arc,  and  by  the 
incandescent  lamp ;  as  the  arc  lamp  preceded  the  incandes- 
cent lamp  historically,  we  will  examine  one  or  two  points 
connected  with  it  first. 

I  have  here  all  that  is  necessary  to  illustrate  the  electric 
arc,  viz.,  two  rods  of  carbon  supported  in  line  with  each 
other,  and  so  mounted  that  they  can  be  approached  or  with- 
drawn. Each  carbon  is  connected  with  one  of  the  poles  of 
the  Edison  dynamo  machine  which  is  supplying  electricity 
to  the  incandescent  lamps  which  illuminate  the  whole  of 


72      DYNAMO   MACHINERY    AND   ALLIED   SUBJECTS. 

this  building.  A  resistance  is  interposed  in  the  circuit  of 
the  lamp,  because  the  electromotive  force  of  the  machine 
is  much  in  excess  of  what  the  lamp  requires.  I  now  ap- 
proach the  carbons,  bring  them  into  contact,  and  again 
separate  them  slightly;  you  observe  that  the  break  does 
not  stop  the  current,  which  forces  its  way  across  the  space. 
I  increase  the  distance  between  the  carbons,  and  you  observe 
the  electric  arc  between  their  extremities ;  at  last  it  breaks, 
having  attained  a  length  of  about  1  inch.  Now  the  current 
has  hard  work  to  cross  this  air  space  between  the  carbons, 
and  the  energy  there  developed  is  converted  into  heat, 
which  raises  the  temperature  of  the  ends  of  the  carbon  be- 
yond any  other  terrestrial  temperature.  There  are  several 
points  of  interest  I  wish  to  notice  in  the  electric  arc.  Both 
carbons  burn  away  in  the  air,  but  there  is  also  a  transference 
of  carbon  from  the  positive  to  the  negative  carbon;  there- 
fore, although  both  waste  away,  the  positive  carbon  wastes 
about  twice  as  fast  as  the  negative.  With  a  continuous 
current,  such  as  we  are  using  now,  the  negative  carbon  be- 
comes pointed,  while  the  positive  carbon  forms  a  crater  or 
hollow;  it  is  this  crater  which  becomes  most  intensely  hot 
and  radiates  most  of  the  light;  hence  the  light  is  not  by  any 
means  uniformly  distributed  in  all  directions,  but  is  mainly 
thrown  forward  from  the  crater  in  the  positive  carbon. 
This  peculiarity  is  of  great  advantage  for  some  purposes, 
such,  for  example,  as  military  or  naval  search  lights;  but  it 
necessitates,  in  describing  the  illuminating  power  of  an  arc 
light,  some  statement  of  the  direction  in  which  the  measure- 
ment was  made.  On  account  of  its  very  high  temperature 
the  arc  light  sends  forth  a  very  large  amount  of  visible 
radiation,  and  is  therefore  very  economical  of  electrical 


SOME   POINTS   IN    ELECTRIC    LIGHTING. 


73 


energy.  For  the  same  reason  its  light  contains  a  very  large 
proportion  of  rays  of  high  refrangibility,  blue  and  ultra 
violet.  I  have  measured  the  red  light  of  an  electric  arc 
against  the  red  of  a  candle,  and  have  found  it  to  be  4,700 
times  as  great,  and  I  have  measured  the  blue  of  the  same 
arc  light  against  the  blue  of  the  same  candle,  and  found  it 
to  be  11,380  times  as  great.  The  properties  of  an  electric 
arc  are  not  those  of  an  ordinary  conductor.  Ohm's  law 
does  not  apply.  The  electromotive  force  and  the  current 
do  not  by  any  means  bear  to  each  other  a  constant  ratio. 
Strictly  speaking,  an  electric  arc  cannot  be  said  to  have  an 
electric  resistance  measurable  in  ohms.  We  will  now  ex- 
amine the  electrical  properties  of  the  arc  experimentally. 
In  the  circuit  with  the  lamp  is  a  Thomson  graded  current 
galvanometer  for  measuring  the  current  passing  in  amperes; 
connected  to  the  two  carbons  is  a  Thomson  graded  poten- 
tial galvanometer  for  measuring  the  difference  of  potential 
between  them  in  volts.  We  have  the  means  of  varying 
the  current  by  varying  the  resistance,  which  I  have  already 
told  you  is  introduced  into  the  circuit.  We  will  first  put 
in  circuit  the  whole  resistance  available,  and  will  adjust 
the  carbons  so  that  the  distance  between  them  is,  as  near 
as  I  can  judge,  £  inch.  We  will  afterwards  increase  the  cur- 
rent and  repeat  the  readings.  The  results  are  given  in 
Table  III. 

TABLE  III. 


Current 
Galvanometer. 

Potential 
Galvanometer. 

Amperes. 

Volts. 

Watts. 

H.  P. 

6.2 

12.0 

9.9 

35 

346 

0.46 

9.3 

12.0 

14.9 

35 

521 

0.70 

11.5 

11.8 

18.4 

34 

626 

0.84 

74      DYNAMO    MACHINERY   AND   ALLIED   SUBJECTS. 

If  the  electrical  properties  of  the  arc  were  the  same  as 
those  of  a  continuous  conductor,  the  volts  would  be  in  pro- 
portion to  the  amperes,  if  correction  were  made  for  change 
of  temperature ;  you  observe  that  instead  of  that  the  poten- 
tial is  nearly  the  same  in  the  two  cases.  We  may  say, 
with  some  approach  to  accuracy,  that,  with  a  given  length 
of  arc,  the  arc  opposes  to  the  current  an  electromotive  force 
nearly  constant,  almost  independent  of  the  current.  This 
was  first  pointed  out  by  Edlund.  If  you  will  speak  of  the 
resistance  of  the  electric  arc,  you  may  say  that  the  resist- 
ance varies  inversely  as  the  current.  Take  the  last  exper- 
iment: by  burning  4  cubic  feet  of  gas  per  hour  we  should 
produce  heat  energy  at  about  the  same  rate.  I  leave  any 
of  you  to  judge  of  the  comparative  illuminating  effects.  It 
is  not  my  purpose  to  describe  the  mechanisms  which  have 
been  invented  for  controlling  the  feeding  of  the  carbons  as 
they  waste  away.  Several  lamps  lent  by  Messrs.  Siemens 
Brothers — to  whom  I  am  indebted  for  the  lamp  and  resist- 
ance I  have  just  been  using — lie  upon  the  table  for  inspec- 
tion. An  electric  arc  can  also  be  produced  by  an  alternate 
current.  Its  theory  may  be  treated  mathematically,  and  is 
very  interesting,  but  time  will  not  allow  us  to  go  into  it. 
I  will  merely  point  out  this:  there  is  some  theoretical  reason 
to  suppose  that  an  alternate  current  arc  is  in  some  measure 
less  efficient  than  one  produced  by  a  continuous  current. 
The  efficiency  of  a  source  of  light  is  greater  as  the  mean 
temperature  of  the  radiating  surface  is  greater.  The  max- 
imum temperature  in  an  arc  is  limited  probably  by  the 
temperature  of  volatilization  of  carbon;  in  an  alternate 
current  arc  the  current  is  not  constant,  therefore  the  mean 
temperature  is  less  than  the  maximum  temperature;  in  a 


SOME  POINTS   IN   ELECTRIC   LIGHTING.  75 

continuous  current  arc,  the  current  being  constant,  the 
mean  and  maximum  temperatures  are  equal,  therefore  in  a 
continuous  current  arc  the  mean  temperature  is  likely  to 
be  somewhat  higher  than  in  an  alternate  current  arc. 

We  will  now  pass  to  the  simpler  incandescent  light. 
When  a  current  of  electricity  passes  through  a  continuous 
conductor,  it  encounters  resistance,  and  heat  is  generated, 
as  was  shown  by  Joule,  at  a  rate  represented  by  the  resist- 
ance multiplied  by  the  square  of  the  current.  If  the  cur- 
rent is  sufficiently  great,  the  heat  will  be  generated  at  such 
a  rate  that  the  conductor  rises  in  temperature  so  far 
that  it  becomes  incandescent  and  radiates  light.  At- 
tempts have  been  made  to  use  platinum  and  platinum- 
iridium  as  the  incandescent  conductor,  but  these  bodies 
are  too  expensive  for  general  use,  and  besides,  refractory 
though  they  are,  they  are  not  refractory  enough  to  stand 
the  high  temperature  required  for  economical  incandescent 
lighting.  Commercial  success  was  not  realized  until  very 
thin  and  very  uniform  threads  or  filaments  of  carbon  were 
produced  and  enclosed  in  reservoirs  of  glass,  from  which 
the  air  was  exhausted  to  the  utmost  possible  limit.  Such 
are  the  lamps  made  by  Mr.  Edison  with  which  this  build- 
ing is  lighted  to-night.  Let  us  examine  the  electrical 
properties  of  such  a  lamp.  Here  is  a  lamp  intended  to 
carry  the  same  current  as  those  overhead,  but  of  half  the 
resistance,  selected  because  it  leaves  us  a  margin  of  electro- 
motive force  wherewith  to  vary  our  experiment.  Into  its 
circuit  I  am  able  to  introduce  a  resistance  for  checking  the 
current,  composed  of  other  incandescent  lamps  for  con- 
venience, but  which  I  shall  cover  over  that  they  may  not 
distract  your  attention.  As  before,  we  have  two  galva- 


76      DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


nometers — one  to  measure  the  current  passing  through  the 
lamp,  the  other  the  difference  of  potential  at  its  terminals. 
First  of  all,  we  will  introduce  ar  considerable  resistance; 
you  observe  that,  although  the  lamp  gives  some  light,  it 
is  feeble  and  red,  indicating  a  low  temperature.  We  take 
our  galvanometer  readings.  We  now  diminish  the  resist- 
ance. The  lamp  is  now  a  little  short  of  its  standard  in- 
tensity; with  this  current  it  would  last  1,000  hours  without 
giving  way.  We  again  read  the  galvanometers.  The  re- 
sistance is  diminished  still  further.  You  observe  a  great 
increase  of  brightness,  and  the  light  is  much  whiter  than 
before.  With  this  current  the  lamp  would  not  last  very 
long.  The  results  are  given  in  Table  IV. 

TABLE  IV. 


Current 
Galvanometer. 

Potential 
Galvanometer. 

Amperes. 

Volts. 

Watts. 

Resistance, 
Ohins. 

5.2 

12.8 

0.38 

87 

14 

97 

6.0 

14.3 

('    11 

41 

18 

93 

11.5 

i       28.4 

0.84 

68 

5? 

81 

There  are  three  things  I  want  you  to  notice  in  these 
experiments:  first,  the  light  is  whiter  as  the  current  in- 
creases; second,  the  quantity  of  light  increases  very  much 
faster  than  the  power  expended  increases;  and  third,  the 
resistance  of  the  carbon  filament  diminishes  as  its  tem- 
perature increases,  which  is  just  the  opposite  of  what  we 
should  find  with  a  metallic  conductor.  This  resistance  is 
given  in  ohms  in  the  last  column.  To  the  second  point, 
which  has  been  very  clearly  put  by  Dr.  Siemens  in  his  British 
Association  address,  I  shall  return  in  a  minute  or  two. 


SOME  POINTS  IN  ELECTRIC  LIGHTING.  77 

The  building  is  this  evening  lighted  by  about  200 
lamps,  each  giving  sixteen  candles'  light  when  75  watts  of 
power  are  developed  in  the  lamp.  To  produce  the  same 
sixteen  candles'  light  in  ordinary  flat  flame  gas  burners 
would  require  between  seven  and  eight  cubic  feet  of  gas  per 
hour,  contributing  heat  to  the  atmosphere  at  the  rate  of 
3,400,000  foot-pounds  per  hour,  equivalent  to  1,250  watts; 
that  is  to  say,  equivalent  gas  lighting  would  heat  the  air 
nearly  seventeen  times  as  much  as  the  incandescent  lamps. 

Look  at  it  another  way.  Practically,  about  eight  of  these 
lamps  take  one  indicated  horse  power  in  the  engine  to  supply 
them.  If  the  steam  engine  were  replaced  by  a  large  gas 
engine  this  1  h.  p.  would  be  supplied  by  25  cubic  feet  of 
gas  per  hour,  or  by  rather  less;  therefore  by  burning  gas 
in  a  gas  engine  driving  a  dynamo,  and  using  the  electricity 
in  the  ordinary  way  in  incandescent  lamps,  we  can  obtain 
more  than  five  candles  per  cubic  foot  of  gas,  a  result  you 
would  be  puzzled  to  obtain  in  10-candle  gas  burners. 
With  arc  lights  instead  of  incandescent  Jamps  many  times 
as  much  light  could  be  obtained. 

At  the  present  time,  lighting  by  electricity  in  London 
must  cost  something  more  than  lighting  by  gas.  Let  us 
see  what  are  the  prospects  of  reduction  of  this  cost.  Be- 
ginning with  the  engine  and  boiler,  the  electrician  has  no 
right  to  look  forward  to  any  marked  and  exceptional 
advance  in  their  economy.  Next  comes  the  dynamo;  the 
best  of  these  are  so  good,  converting  80  per  cent,  of  the 
work  done  in  driving  the  machine  into  electrical  work  out- 
side the  machine,  that  there  is  little  room  for  economy  in 
the  conversion  of  mechanical  into  electrical  energy;  but 
the  prime  cost  of  the  dynamo  machine  is  sure  to  be  greatly 


78  DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

reduced.  Our  hope  of  greatly  increased  economy  must  be 
mainly  based  upon  probable  improvements  in  the  incan- 
descent lamp,  and  to  this  the  greatest  attention  ought  to 
be  directed.  You  have  seen  that  a  great  economy  of 
power  can  be  obtained  by  working  the  lamps  at  high  press- 
ure, but  then  they  soon  break  down.  In  ordinary  prac- 
tice from  140  to  200  candles  are  obtained  from  a  horse 
power  developed  in  the  lamps,  but  for  a  short  time  I  have 
seen  over  1,000  candles  per  horse  power  from  incandescent 
lamps.  The  problem,  then,  is  so  to  improve  the  lamp  in 
detail  that  it  will  last  a  reasonable  time  when  pressed  to 
that  degree  of  efficiency.  There  is  no  theoretical  bar  to 
such  improvements,  and  it  must  be  remembered  that  in- 
candescent lamps  have  only  been  articles  of  commerce  for 
about  three  years,  and  already  much  has  been  done.  If 
such  an  improvement  were  realized,  it  would  mean  that 
you  would  get  five  times  as  much  light  for  a  sovereign  as 
you  can  now.  As  things  now  stand,  so  soon  as  those  who 
supply  electricity  have  reasonable  facilities  for  reaching 
their  customers,  electric  lighting  will  succeed  commercially 
where  other  considerations  than  cost  have  weight.  We 
are  sure  of  some  considerable  improvements  in  the  lamps, 
and  there  is  a  probability  that  these  improvements  may 
go  so  far  as  to  reduce  the  cost  to  one-fifth  of  what  it  now 
is.  I  leave  you  to  judge  whether  or  not  it  is  probable,  nay, 
almost  certain,  that  lighting  by  electricity  is  the  lighting 
of  the  future. 


MAOHlNEKY. 


DYNAMO-ELECTKIC  MACHINEKY. 

THEORETICAL   CONSTRUCTION"   OF   CHARACTERISTIC   CURVE. 

OMITTING  the  inductive  effects  of  the  current  in  the 
armature  itself,  all  the  properties  of  a  dynamo  machine  are 
most  conveniently  deduced  from  a  statement  of  the  rela- 
tion between  the  magnetic  field  and  the  magnetizing  force 
required  to  produce  that  field,  or,  which  comes  to  the  same 
thing  but  more  frequently  used  in  practice,  the  relation 
between  the  electromotive  force  of  the  machine  at  a  stated 
speed  and  the  current  around  the  magnets.  This  relation 
given,  it  is  easy  to  deduce  what  the  result  will  be  in  all 
employments  of  the  machine,  whether  as  a  motor  or  to 
produce  a  current  through  resistance,  through  an  electric 
arc,  or  in  charging  accumulators;  also  the  result  of  vary- 
ing the  winding  of  the  machine,  whether  in  the  armature 
or  magnets.  The  proper  independent  variable  to  choose 
for  discussing  the  effect  of  a  dynamo  machine  is  the  cur- 
rent around  the  magnets;  and  the  primary  relation  it  is 
necessary  to  know  concerning  the  machine  is  the  relation 
of  the  electromotive  force  of  the  armature  to  the  magnet 
current.  This  primary  relation  may  be  expressed  by  a 
curve  (Fig.  4,  p.  22  et  seq.,  and  Fig.  5,  p.  26),  now  called 
the  characteristic  of  the  machine,  and  all  consequences 
deduced  therefrom  graphically;  or  it  may  be  expressed  by 


80    DYNAMO   MACHINERY   AND  ALLIED  SUBJECTS. 

stating  the  E.M.F.  as  an  empirical  function  of  the  magnet- 
izing current.  Many  such  empirical  formulaB  have  been 
proposed;  as  an  instance  we  may  mention  that  known  as 
Frohlich's,  according  to  whom,  if  c  be  the  current  in  the 

magnets,  E  the  resulting  E.M.F.,  E  =  —  ~r-.    For  some 

J.  — J~  o  c  • 

machines  this  formula  is  said  to  express  observed  results 
fairly  accurately,  but  in  our  experience  it  does  not  suf- 
ficiently approximate  to  a  straight  line  in  the  part  of  the 
curve  near  the  origin.  The  character  of  the  error  in 
Frohlich's  formula  is  apparent  by  reference  to  Figs.  24 
and  25,  which  give  a  series  of  observations  on  a  dynamo 
machine,  and  for  comparison  therewith  a  hyperbola  F, 
drawn  as  favorably  as  possible  to  accord  with  the  observa- 
tions.* Such  empirical  formulaB  possess  no  advantage  over 
the  graphical  method  aided  by  algebraic  processes,  and 
tend  to  mask  much  that  is  of  importance. 

One  purpose  of  the  present  investigation  is  to  give  an 
approximately  complete  construction  of  the  characteristic 
curve  of  a  dynanlo  of  given  form  from  the  ordinary  laws 
of  electromagnetism  and  the  known  properties  of  iron, 
and  to  compare  the  result  of  such  construction  with  the 
actual  characteristic  of  the  machine.  The  laws  of  electro- 
magnetism  needed  are  simply  (Thomson,  papers  on  "  Elec- 

*  Added  Aug.  17.— That  Frfihlich's  formula  cannot  be  a  thoroughly  satisfac- 
tory expression  of  the  characteristic  of  a  dynamo  machine  is  evident  from  the 
consideration  that  E  should  simply  change  its  sign  with  c,  that  is,  be  an  odd 
function  of  c.  There  should  be  a  point  of  inflection  in  the  characteristic  curve 

at  the  origin.  Another  empirical  formula,  —  =  tan  ~  1  -,  is  free  from  this  objec- 
tion, but  still  fails  to  fully  represent  the  approximation  of  the  curve  to  a 
straight  line  on  either  side  of  the  origin,  and  it  is  equally  uninstructive  with 
any  other  purely  empirical  formula. 


DYNAMO-ELECTEIC   MACHINERY 


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results,  -f  ascending,  ®  descending;    F,  Frfihlich's  curve.    This  figure  is 
the  same  as  the  left-hand  part  of  Fig.  24,  but  on  a  larger  scale. 


DYNAMO-ELECTRIC   MACHINERY.  83 

trostatics  and  Magnetism;"  Maxwell,  "Electricity  and 
Magnetism/'  vol.  ii.,  pp.  24,  26,  and  143),  (1)  that  the 
line  integral  of  magnetic  force  around  any  closed  curve, 
whether  in  iron,  in  air,  or  in  both,  is  equal  to  4;r  n  c,  where 
c  is  the  current  passing  through  the  closed  curve,  and  n  is 
the  number  of  times  it  passes  through;  (2)  the  solenoidal 
condition  for  magnetic  induction,  that  is,  if  the  lines  of 
force  or  of  induction  be  supposed  drawn,  then  the  induc- 
tion through  any  tube  of  induction  is  the  same  for  every 
section.  Regarding  the  iron  itself,  we  require  to  know 
from  experiments  on  the  material  in  any  shape  the  relation 
between  «,  the  induction,  and  a,  the  magnetic  force  at  any 
point;  for  convenience  write  a=f~l(ae),  or  a  =  f(a). 
From  these  premises,  without  any  further  assumption,  it  is 
easy  to  see  that  a  sufficiently  powerful  and  laborious  analysis 
would  be  capable  of  deducing  the  characteristic  of  any 
dynamo  to  any  desired  degree  of  accuracy.  This  we  do  not 
attempt,  as,  even  if  successful,  the  analysis  would  not  be 
likely  to  throw  any  useful  light  on  the  practical  problem. 
We  shall  calculate  the  characteristic,  first  making  certain 
assumptions  to  simplify  matters.  We  shall  next  point  out 
the  nature  of  the  errors  introduced  by  these  assumptions, 
and  make  certain  small  corrections  in  the  method  to  ac- 
count for  these  sources  of  error,  merely  proving  that  the 
amount  of  these  corrections  is  probable  or  deducing  it 
from  a  separate  experiment,  and  again  compare  the  theo- 
retical and  the  actual  characteristic. 

First  Approximation. — Assume  that  by  some  miracle 
the  tubes  of  magnetic  induction  are  entirely  confined  to 
the  iron  excepting  that  they  pass  directly  across  from  the 
bored  faces  of  the  pole  pieces  to  the  cylindrical  face  of  the 


84  DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

armature  core.  This,  we  shall  find,  introduces  minor 
sources  of  error,  affecting  different  parts  of  the  charac- 
teristic curve  to  a  material  extent.  Let  /  be  total  in- 
duction through  the  armature,  Al  the  area  of  section  of 
the  iron  of  the  armature,  7,  the  mean  length  of  lines  of 
force  in  the  armature;  .1,  the  area  of  each  of  the  two 
spaces  between  core  of  armature  and  the  pole  pieces  of  the 
magnets,  7,  the  distance  between  the  core  and  the  pole 
piece;  At  the  area  of  core  of  magnet,  7,  the  total  length  of 
the  magnets.  All  the  tubes  of  induction  which  pass 
through  the  armature  pass  through  the  space  A9  and  the 
magnet  cores,  and  by  our  assumption  there  are  no  others. 
We  now  assume  further  that  these  tubes  are  uniformly 
distributed  over  these  areas.  The  induction  per  square 

centimetre  is  then  -.-  in  the  armature  core,  — -  in  the  non- 
Al  A9 

magnetic  spaces,  --  in  the  magnet  cores;  the  correspond- 
ing magnetic  forces  per  centimetre  linear  must  be/(  -  -),  -y-, 

\AJ  At 

The  line  integral  of  magnetic  force  round  a  closed 

curve  must  be  7,/f-;- )  +  27,  -  -  +  J«/(~r )•     ^n  this  aP- 
XAJ  At  \AtJ 

proximation  we  neglect  the  force  required  to  magnetize 
pole  pieces  and  other  parts  not  within  the  magnet  coils,  to 
avoid  complication.  The  equation  of  the  characteristic 

curve  is,  then,  4*  n  c  =  7,/f—l  +  27,  -.-  +  7,  f(-\    This 

\AJ  A9  \A3/ 

curve  is,  of  course,  readily  constructed  graphically  from 
the  magnetic  property  of  the  material  expressed  by  the 


DYNAMO-ELECTRIC   MACHINERY. 


85 


curve  a  =f(a).     In  Figs.  24  and  25  curve  A  represents 

x.=  Z,  f[  -J-),   the    straight  line   B  x  =  2Z,-j-,   curve     G 
\AJ  J±i 

x  =  Z,  /(•?'),  and  curve  D  the  calculated   characteristic. 


When  we  compare  this  with  an  actual  characteristic  E,  we 
shall  see  that,  broadly  speaking,  it  deviates  from  truth  in 


FIG.  26. 

two  respects:  (1)  it  does  not  rise  sufficiently  rapidly  at 
first;  (2)  it  attains  a  higher  maximum  than  is  actually 
realized.  Let  us  examine  these  errors  in  detail. 

(1)  The  angle  the  characteristic  makes  with  the  axis  of 
abscissae  near  the  origin  is  mainly  determined  by  the  line 
B  (Fig.  26).  "We  have  in  fact  a  very  considerable  exten- 


86     DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

sion  of  the  area  of  the  field  beyond  that  which  lies  under 
the  bored  face  of  the  pole  piece.  The  following  considera- 
tion will  show  that  the  extension  may  be  considerable: 
Imagine  an  infinite  plane  slab,  and  parallel  with  it  a  second 
slab  cut  off  by  a  second  plane  making  an  angle  a.  We 
want  a  rough  idea  of  the  extension  of  the  area  between  the 
plates  by  the  spreading  of  the  lines  of  induction  beyond 
the  boundary.  We  know  that  the  actual  extension  of  the 
area  will  be  greater  than  we  shall  calculate  it  to  be  if  we 
prescribe  an  arbitrary  distribution  of  lines  of  force  other 
than  that  which  is  consistent  with  Laplace's  equation. 
Assume,  then,  the  lines  of  force  to  be  segments  of  circles 

centre  0,  and  straight  lines  perpendicular  to  0  A.    The 

y 
induction   along  a  line   P  Q  R  will  be  -T—     — r—  —7-,  V 

t 

being  difference  of  potential  between  the  planes;  and  the 
added  induction  from  0  P  B  will  be 

Vdx  V  (T-  a)x  +  t 

(it  -  a)x  4-  t  ~~  n  -  a     g  "  t 

7t 

Thus,  if  n  =  — ,  we  have  for  x  =  t,  21,  etc., 


t 
2t 


a)x  -f  t 

lUi^ 

TT  —  a 
0.599 
0.904 
1.109 
1.263 
,  1.387 
1.793 

t 

V   IS 


DYNAMO-ELECTRIC   MACHINERY.  87 

showing  that  the  extension  of  the  area  of  the  field  is  likely 
to  be  considerable. 

(2)  The  failure  of  the  actual  curve  to  reach  the  max- 
imum indicated  by  approximate  theory  is  because  the 
theory  assumes  that  all  tubes  of  induction  passing  through 
the  magnets  pass  also  through  the  armature.  Familial- 
observations  round  the  pole  pieces  of  the  magnets  show 
that  this  is  not  the  case.  If  v  be  the  ratio  of  the  total 
induction  through  the  magnets  to  the  induction  in  the 
armature,  we  must,  in  our  expression  for  the  line  integral  of 

magnetizing  force,  replace  the  term  /f-j-J  by  /( ~  J  : 

not  strictly  a  constant,  as  we  shall  see  later;  it  is  somewhat 
increased  as  /  increases,  owing  to  magnetization  of  the  core 
of  the  armature,  and  it  is  also  affected  by  the  current  in 
the  armature.  For  our  present  purpose  we  treat  it  as  con- 
stant. 

There  is  yet  another  source  of  error  which  it  is  necessary 
to  examine.  Some  part  of  the  induction  in  the  armature 
may  pass  through  the  shaft  instead  of  through  the  iron 
plates.  An  idea  of  the  amount  of  this  disturbance  may  be 
readily  obtained.  Consider  the  closed  curve  A  B  C  D  E  F: 
A  B  and  FED  C  are  drawn  along  lines  of  force;  A  F  and 
B  C  are  orthogonal  to  lines  of  force  (Fig.  27).  Since  this 
closed  curve  has  no  currents  passing  through  it,  the  line 
integral  of  force  around  it  is  nil;  therefore,  neglecting 
force  along  E  D,  we  have  force  along  A  B  equal  to  force 
along  F  E  and  D  C.  In  the  machine  presently  described 
we  may  safely  neglect  the  induction  through  the  shaft; 
the  error  is  comparable  with  the  uncertainty  as  to  the  value 
of  ^;  but  in  another  machine, '  with  magnets  of  much 


88    DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


greater  section,  the  effect  of  the  shaft  would  become  very 
sensible  when  the  core  is  practically  saturated. 


Fio.  27. 

The  amended  formula  now  becomes 


n  c  =  1 .  1 1 


where  14  is  the  mean  length  of  lines  of  force  in  the 
wrought-iron  yoke,  A4  the  area  of  the  yoke,  lt  and  At  cor- 
responding quantities  for  the  pole  pieces,  the  last  two 
terms  being  introduced  for  the  forces  required  to  magnet- 
ize the  yoke  and  the  two  pole  pieces. 


DYNAMO-ELECTRIC   MACHINERY.  89 

"We  now  repeat  the  graphical  method  of  construction 
exactly  as  before,  the  actual  observations  of  induction  in 
armature  and  current  being  plotted  on  the  same  diagram, 
Figs.  28  and  29,  in  which  curve  G  represents  the  force 
required  to  magnetize  the  yoke,  and  curve  H  that  required 
to  magnetize  the  pole  pieces.  Before  discussing  these 
curves  further,  and  comparing  the  results  with  those  of 
actual  observation,  it  may  be  convenient  to  describe  the 
machine  upon  which  the  experiments  have  been  made, 
confining  the  description  strictly  to  so  much  as  is  perti- 
nent to  our  present  inquiry. 

DESCRIPTION   OF   MACHINE. 

The  dynamo  has  a  single  magnetic  circuit,  consisting  of 
two  vertical  limbs,  extended  at  their  lower  extremities  to 
form  the  pole  pieces,  and  having  their  upper  extremities 
connected  by  a  yoke  of  rectangular  section.  Each  limb, 
together  with  its  pole  piece,  is  formed  of  a  single  forging 
of  wrought  iron.  These  forgings,  as  also  that  for  the  yoke, 
are  built  up  of  hammered  scrap  and  afterwards  carefully 
annealed,  and  have  a  magnetic  permeability  but  little  in- 
ferior to  the  best  Swedish  charcoal  iron.  The  yoke  is 
held  to  the  limbs  by  two  bolts,  the  surfaces  of  contact 
being  truly  planed.  In  section  the  limb  is  oblong,  with 
the  corners  rounded  in  order  to  facilitate  the  winding  of 
the  magnetizing  coils.  A  zinc  base,  bolted  to  the  bed- 
plate of  the  machine,  supports  the  pole  pieces. 

The  magnetizing  coils  are  wound  directly  on  the  limbs, 
and  consist  of  11  layers  on  each  limb,  of  copper  wire  2.413 
mms,  diameter  (No.  13,  B.W.Gr.),  making  3,260  convolu- 


90     DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


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DYNAMO-ELECTRIC   MACHINERY. 


91 


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5000                       10000                      15000                      20000                      25000    force 

Fio.  29. — CORRECT  SYNTHESIS  OP  CHARACTERISTIC  CURVE. 
A,  armature;    B,  air  space;    C,  magnets;    D,  calculated  curve;    £7.  observa- 
tions,-(-ascending,  ®  descending;    (?,  yoke;    H,  pole  piece.    This  figure 
is  the  same  as  the  left-hand  part  of  Fig.  28,  but  on  a  larger  scale, 


92     DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

tions  in  all,  the  total  length  being  approximately  4,570 
metres.  The  pole  pieces  are  bored  to  receive  the  arma- 
ture, leaving  a  gap  above  and  below,  subtending  an  angle 
of  51°  at  the  centre  of  the  fields.  The  opposing  surfaces 
of  the  gap  are  8  mms.  deep. 

The  following  table  gives  the  leading  dimensions  of  the 
machine: 

cms. 

Length  of  magnet  limb =  45.7 

Width  of  magnet  limb =  22.1 

Breadth  of  magnet  limb =  44.45 

Length  of  yoke =  «il  6 

Width  of  yoke        =  48.3 

Depth  of  yoke =  23  2 

Distances  between  ceni  res  of  limbs =  88.1 

Bore  of  fields  =  27.5 

Depth  of  pole  piece        '. =  25  4 

Width  of  pole  piece  measured  'parallel  to  the  shaft =  48.3 

Thickness  of  zinc  base  =  12.7 

Width  of  gap =  12.7 

The  armature  is  built  up  of  about  1,000  iron  plates,  insu- 
lated one  from  another  by  sheets  of  paper,  and  held  between 
two  end  plates,  one  of  which  is  secured  by  a  washer 
shrunk  on  to  the  shaft,  and  the  other  by  a  nut  and  lock- 
nut  screwed  on  the  shaft  itself.  The  plates  are  cut  from 
sheets  of  soft  iron,  having  probably  about  the  same  mag- 
netic permeability  as  the  magnet  cores.  The  shaft  is  of 
Bessemer  steel,  and  is  insulated  before  the  plates  are 
threaded  on. 

The  following  table  gives  the  leading  dimensions  of  the 
armature : 

cms. 

Diameter  of  core =  24.5 

Diameter  of  internal  hole =   7.62 

Length  of  core  over  the  end  plates =  50.8 

Diameter  of  shaft =  6,985 


DYNAMO-ELECTRIC   MACHINERY. 


93 


The  core  is  wound  longitudinally  according  to  the 
Hefner  von  Alteneck  principle  with  40  convolutions,  each 
consisting  of  16  strands  of  wire  1.753  inm.  diameter,  the 
convolutions  being  placed  in  two  layers  of  20  each.  The 
commutator  is  formed  of  40  copper  bars,  insulated  with 
mica,  and  the  connections  to  the  armature  so  made  that 
the  plane  of  commutation  in  the  commutator  is  horizontal 
when  no  current  is  passing  through  the  armature. 

Fig.  30  shows  a  side  elevation  of  the  dynamo;  Fig*  31  a 


FIG   30. 

cross  section  through  the  centres  of  the  magnets;  Fig  32 
a  section  of  the  core  of  the  armature,  in  a  plane  through 
the  axis  of  the  shaft. 

The  dynamo  is  intended  for  a  normal  output  of  105 
volts  320  amperes  at  a  speed  of  750  revolutions  per  minute. 


94     DYNAMO  MACHINERY    AND  ALLIED   SUBJECTS. 

The  resistance  of  the  armature  measured  between  opposite 
bars  of  the  commutator  is  0.009947  ohm,  and  of  the  mag- 


Fio.  81. 


net  coils  16.93  ohms,  both  at  a  temperature  of  13.5°  Centi- 
grade; Lord  Rayleigh's  determination  of  the  ohm  being 
assumed. 


Fio.  32. 


We  have  now  to  estimate  the  lengths  and  areas  required 
in  the  synthesis  of  the  characteristic  curve. 

-4,; — from  the  length  of  the  core  of  the  armature  (50.8 
cms.)  must  be  deducted  3.4  cms.  for  the  thickness  of 


DYHAMO-ELECTKIC  MACHINERY.  95 

insulating  material  between  the  plates ;  the  resultant  area 
is,  on  the  other  hand,  as  has  already  been  stated,  slightly 
augmented  by  the  presence  of  the  steel  shaft.  A1  is  taken 
as  810  sq.  cms. 

/! ; — this  is  assumed  to  be  13  cms.,  i.e.,  slightly  in  excess 
of  the  shortest  distance  (12.6  cms.)  between  the  pole  pieces. 

A^j — the  angle  subtended  by  the  bored  face  of  the  pole 
piece  at  the  axis  is  129°,  the  breadth  of  the  pole  piece  is 
48.3  cms.,  the  diameter  of  the  bore  of  the  field  is  27.5  cms., 
and, as  already  stated,  the  diameter  of  core  24.5  cms.;  thus 
the  area  of  pole  piece  is  1,513  sq.  cms.,  and  the  area  of  129° 
of  the  cylinder  at  the  mean  radius  of  13.0  cms.  is  1,410  sq. 
cms. ;  this  value  is  taken  for  A^  in  the  curves  drawn  in  Figs. 
24  and  25.  In  Figs.  28  and  29  A^  is  taken  as  1,600,  an 
allowance  of  190  sq.  cms.  being  made  for  the  spreading  of 

the  field  at  the  edges  of  the  pole  pieces,  or  -—-  =  1.2  cm.  all 

1  2 

round  the  periphery,  that  is,  -^=  0.8  of  the  distance  from 

l.o 

iron  of  pole  pieces  to  iron  of  core. 

19  is  1.5  cm. 

A9  is  a  little  uncertain,  as  the  forgings  are  not  tooled 
all  over;  it  is  here  taken  as  980  sq.  cms., 
but  this  value  may  be  slightly  too  high. 

ls  is  91.4  cms. 

A4  is  1,120  sq.  cms, 

14  is  49  cms.,  being  measured  along  a 
quadrant  from  the  centre  of  the  magnet 
(see  Fig.  33).  FIG.  33. 

AI  is  1,230  sq.  cms.,  intermediate  between  the  area  of 
magnet  and  face  of  pole  piece. 


96     DYNAMO   MACHINERY  AND  ALLIED   SUBJECTS. 

16  is  11  cms. 

v  was  determined  by  experiment  as  described  below,  and 
its  value  is  taken  as  1.32;  when  the  magnetizing  current  is 
more  than  5.62  amperes  its  value  should  be  a  little  greater. 

The  function/(rt)  is  taken  from  Hopkinson,  Phil.  Trans., 
vol.  clxxvi,  1885,  p.  455 ;  the  wrought  iron  there  referred 
to  was  not  procured  at  the  same  time  as,  and  its  properties 
may  differ  to  a  certain  extent  from,  the  wrought  iron  of 
these  magnets. 

The  curves  now  explain  themselves :  the  abscissas  in  each 
case  represent  the  line  integral  of  magnetizing  force  in  the 
part  of  the  magnetic  circuit  referred  to;  the  ordinates,  the 
number  of  lines  of  induction  which  also  pass  through  the 
armature. 

The  results  of  the  actual  observations  on  the  machine 
are  indicated,  those  when  the  magnetizing  force  is  increas- 
ing +>  when  it  is  decreasing  ®.  The  measurements  of  the 
currents  in  the  magnets  which  were  separately  excited,  and 
of  the  potential  difference  between  the  bmshes,  the  circuit 
being  open,  were  made  with  Sir  W.  Thomson's  graded 
galvanometers,  standardized  at  the  time  of  use.  The  irreg- 
ularities of  the  observations  are  probably  due  to  the  varia- 
tion of  speed,  the  engine  being  not  quite  perfectly  governed. 
The  second  construction  exhibits  quite  as  close  an  agree- 
ment between  observation  and  calculation  as  could  be 
expected;  the  deviation  at  high  magnetizing  forces  is 
probably  due  to  three  causes — increase  in  the  value  of  v 
when  the  core  of  the  armature  is  partially  saturated,  un- 
certainty as  to  the  area  J3,  difference  in  the  quality  of  the 
iron.  It  is  interesting  to  see  how  clearly  theory  predicts  - 
the  difference  between  the  ascending  and  descending  curves 


DYNAMO-ELECTRIC   MACHINERY.  97 

of  a  dynamo.  Consideration  of  the  diagram  proves  that 
this  machine  is  nearly  perfect  in  its  magnetic  proportions. 
The  core  might  be  diminished  without  detriment  by  in- 
creasing the  hole  through  it  to  a  small,  but  very  small, 
extent.  Any  reduction  of  area  of  magnets  would  be  inju- 
rious ;  they  might,  indeed,  be  slightly  increased  with  advan- 
tage. An  increase  in  the  length  of  the  magnets  would  be 
very  distinctly  detrimental.  Again,  little  advantage  results 
from  increasing  the  magnetizing  force  beyond  the  point  at 
which  the  permeability  of  the  iron  of  the  magnets  begins 
to  rapidly  diminish.  For  iron  of  the  same  quality  as  that  of 
the  machine  under  consideration,  a  magnetizing  force  of 
2.6x10'  or  28.4  per  centimetre  is  suitable.  To  get  the 
same  induction  in  other  parts  of  the  circuit,  the  diagram 
shows  that  for  the  air  space  a  magnetizing  force  of  21 X  10s 
is  required,  for  the  pole  pieces  0.1  X 108,  for  the  armature 
0.2  XlO3,  for  the  yoke  0.6X108;  making  a  total  force  re- 
quired of  24.5  X 10".  Any  alteration  in  the  length  of  the 
area  of  any  portion  of  the  magnetic  circuit  entails  a  corre- 
sponding alteration  in  the  magnetizing  forces  required  for 
that  portion,  at  once  deducible  from  the  diagram.  Similar 
machines  must  have  the  magnetizing  forces  proportional 
to  the  linear  dimensions,  and  consequently,  if  the  electro- 
motive force  of  the  machines  is  the  same,  the  diameter  of 
the  wire  of  the  magnet  coils  must  be  proportional  to  the 
linear  dimensions.  If  the  lengths  of  the  several  portions 
of  the  magnetic  circuit  remain  the  same,  but  the  areas  are 
similarly  altered,  the  section  of  the  wire  must  be  altered  in 
proportion  to  the  alteration  in  the  periphery  of  the  section. 


DYNAMO   MACHINERY   AND   ALLIED  SUBJECTS. 


EXPERIMENT  TO   DETERMINE   V. 

Around  the  middle  of  one  of  the  magnet  limbs  a  single 
coil  of  wire  was  taken,  forming  one  complete  convolution, 
and  its  ends  connected  to  a  Thomson's  mirror  galvanom- 
eter rendered  fairly  ballistic.  If  the  circuit  of  the  field 
magnets,  while  the  exciting  current  is  passing,  be  suddenly 
short  circuited,  the  elongation  of  the  galvanometer  is  a 
measure  of  the  total  induction  within  the  core  of  the 
limbs,  neglecting  the  residual  magnetization.  If  the  short 
circuit  be  suddenly  removed,  so  that  the  current  again 
passes  round  the  field  magnets,  the  elongation  of  the 
galvanometer  will  be  equal  in  magnitude  and  opposite  in 
direction. 

The  readings  taken  were : 

Zero 71  left. 

Deflection    ....     332    "   magnets  made. 

"  ....     196  right;  magnets  short  circuited, 

llence,  deflection  to  right  =  267 
left    =  261 
Mean  deflection  =  264 

To  determine  the  induction  through  the  armature,  the 
leads  to  the  ballistic  galvanometer  were  soldered  to  con- 
secutive bars  of  the  commutator,  connected  to  that  convo- 
lution of  the  armature  which  lay  in  the  plane  of  commu- 
tation. 


DYNAMO-ELECTKIC   MACHINERY.  99 

The  readings  taken  were : 

Zero 23    left. 

Deflection  ....     223      "  magnets  made. 

'    '     '     '  j-  right ;  magnets  short  circuited. 

Hence,  deflection  to  right  and  left  =  200 

It  thus  appears  that  out  of  264  lines  of  force  passing 
through  the  cores  of  the  magnet  limbs  at  their  centre,  200 
go  through  the  core  of  the  armature,  whence  v  equals  1.32. 
The  magnetizing  current  round  the  fields  during  these 
experiments  was  5.G  amperes. 

EXPERIMENTS   ON"   WASTE    FIELD    NOT    PASSING    THROUGH 
ARMATURE. 

As  in  the  determination  of  v,  a  single  convolution  was 
taken  around  the  middle  of  one  of  the  limbs,  and  con- 
nected to  the  ballistic  galvanometer;  the  deflections,  when 
a  current  of  5.6  amperes  was  suddenly  passed  through  the 
fields  or  short  circuited,  were : 

Zero 34  left. 

Deflection    ....     148     "   magnets  made. 

"  ....       82  right;  magnets  short  circuited. 

Hence,  deflection  to  right  =116 
left     =  114 
Mean  deflection  =  115 

I.  Four  convolutions  were  then  wound  round  the  zinc 
plate  and  the  cast-iron  bed  in  a  vertical  plane,  passing 


100   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

through  the  axis  of  the  armature;   and  the  deflections 
noted  were: 

Zero.    .'  .    .    ...    .     15  left. 

Deflection 61    "  magnets  short  circuited. 

"         40  right;  magnets  made. 

Zero 11  left. 

Deflection 64     "   magnets  short  circuited. 

"         36  right;  magnets  made. 

Hence,  deflection  to  right  =  55 
left     =  46 

and         "  right  =  47 

left     =53 

in  the  two  observations  respectively,  giving  a  mean  =  50.25; 
or,  reducing  to  one  convolution,  =  12.6. 

II.  A  square  wooden  frame,  38  cms.  x  38  cms.,  on  which 
were  wound  ten  convolutions,  was  then  inserted  between 
the  magnet  limbs,  with  one  side  resting  on  the  armature, 
and  an  adjacent  side  projecting  5  cms.  beyond  the  coils  on 
the  limbs,  or  about  7.6  cms.  beyond  the  cores  of  the  limbs. 
The  deflections  were: 

Zero 34  left. 

Deflection 98    "   magnets  made. 

"         22  right;  magnets  short  circuited. 

«  21     "  <s  " 

"         81  left;  magnets  made. 

Hence,  deflection  to  right  =  56 
left     =  64 

and  "  right  =  55 

left     =  47 


DYNAMO-ELECTRIC    MACHINERY.  101 

in  the  two  observations  respectively,  giving  a  mean  =  55  ; 
or,  reducing  to  one  convolution,  =  5.5. 

III.  The  same  frame  was  raised  a  height  of  6.35  cms. 
above  the  armature  in  a  vertical  plane.     The  deflections 
were: 

Zero  ......      21  left. 

Deflection  ....      98    "    magnets  made. 

Zero  ......       35  left. 

Deflection  ....        8  right;  magnets  short  circuited. 

Hence,  deflection  to  left     =  50 
right  =  43 

and  mean  deflection  =  46.5 
or,  reducing  to  one  convolution,  =    4.6 

IV.  The  same  frame  was  again  lowered  on  the  armature 
and  pushed  inwards  so  as  to  lie  symmetrically  within  the 
space  between  the  limbs.     The  deflections  were  : 

Zero      ......       32  right. 

Deflection      ....     112      "     magnets  made. 

"  ....       48  left;  magnets  short  circuited. 

Giving  a  mean  of  80;    or,  reducing  to  one  convolution, 
=  8.0. 

Let  G  represent  the  leakage  through  a  vertical  area 
bounded  by  the  armature  and  a  line  7.6  cms.  above  the 
armature  and  of  the  same  width  as  the  pole  pieces;  let  R 
be  the  remainder  of  the  leakage  between  the  limbs;  then 
II.  and  III.  give 


f  *  =  4.6; 


102   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

whence 

G  =  1.35, 
R  =  6.9. 
Again,  IV.  gives 

|(fl<  +  *)  =  8.0; 

therefore 

G  +  R  =  9.6, 

which  shows  an  agreement  as  near  as  might  be  expected 
considering  the  rough  nature  of  the  experiment  and  that 
the  leakage  is  assumed  uniform  over  the  areas  considered. 
We  take 

G  =  1.6, 
R  =  8.0. 

Reducing  these  losses  to  percentages  we  have 

i  c 

G  =  T       —     1.4  per  cent. 

lio 

*-ra -»    ' 

And  from  I.  the  leakage  through  the  )  .103  « 

zinc  plate  and  iron  base    .     .     .     .  ) 

Hence  the  two  gaps  account  for  .     .     .  2.8  " 

The  zinc  plate  and  iron  base  account  for  10.3  " 

And  the  area  between  the  limbs  .     .     .  7.0  " 

Making  a  total  loss  accounted  for     .     .  20.1  " 

Out  of  an  observed  loss  of 24.24  " 

The  leakage  through  the  shaft  and  from  pole  piece  to 
yoke,  and  one  pole  piece  to  the  other  by  exterior  lines, 
will  account  for  the  remainder. 


DYNAMO-ELECTRIC    MACHINERY.  103 


EFFECT   OF   THE   CURRENT   IN   THE   ARMATURE. 

The  currents  in  the  fixed  coils  around  the  magnets  are 
not  the  only  magnetizing  forces  applied  in  a  dynamo 
machine ;  the  currents  in  the  moving  coils  of  the  armature 
have  also  their  effect  on  the  resultant  field.  There  are  in 
general  two  independent  variables  in  a  dynamo  machine — 
the  current  around  the  magnets  and  the  current  in  the 
armature;  and  the  relation  of  E.M.F.  to  currents  is  fully 
represented  by  a  surface.  In  well-constructed  machines 
the  eifect  of  the  latter  is  reduced  to  a  minimum,  but  it 
can  be  by  no  means  neglected.  When  a  section  of  the 
armature  coils  is  commutated,  it  must  inevitably  be 
momentarily  short  circuited;  and  if  at  the  time  of  commu- 
tation the  field  in  which  the  section  is  moving  is  other 
than  feeble,  a  considerable  current  will  arise  in  that  see- 
tion,  accompanied  by  waste  of  power  and  destructive 
sparking.  It  may  be  well  at  once  to  give  an  idea  of  the 
possible  magnitude  of  such  effects.  In  the  machine  al- 
ready described  the  mean  E.M.F.  in  a  section  of  the  arma- 
ture at  a  certain  speed  may  be  taken  as  6  volts,  its  resist- 
ance 0.000995  ohm.  Setting  aside,  then,  for  the  moment 
questions  of  self  induction,  if  a  section  were  commutated 
at  a  time  when  it  was  in  a  field  of  one  tenth  part  of  the 
mean  intensity  of  the  whole  field,  there  would  arise  in  that 
section,  while  short  circuited  by  the  collecting  brush,  a 
current  of  600  amperes,  four  times  the  current  when  the 
section  is  doing  its  normal  work.  The  ideal  adjustment 
of  the  collecting  brushes  is  such  that  during  the  time  they 
short  circuit  the  sections  of  the  armature  the  magnetic 


104   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

forces  shall  just  suffice  to  stop  the  current  in  the  section, 
and  to  reverse  it  to  the  same  current  in  the  opposite  direc- 
tion. 

Suppose  the  commutation  occurs  at  an  angle  A  in  ad- 
vance of  the  symmetrical  position  between  the  fields,  and 


Fio.  34. 

that  the  total  current  through  the  armature  be  (7,  reckoned 
positive  in  the  direction  of  the  resultant  E.M.F.  of  the 
machine,  i.e.,  positive  when  the  machine  is  used  as 
a  generator  of  electricity.  Taking  any  closed  line 
through  magnets  and  armature,  symmetrically  drawn  as 
AB  C D  E  FA  (Fig.  34),  it  is  obvious  that  the  line  in- 
tegral of  magnetic  force  is  diminished  by  the  current  in 
the  armature  included  between  angle.  A  in  front  and  angle 


DYNAMO-ELECTRIC   MACHINERY.  105 

A  behind  the  plane  of  symmetry.  If  m  be  the  number  of 
convolutions  of  the  armature,  the  value  of  this  magnetizing 

force  is  ±n  C^ —  =  £kmC  opposed  to  the  magnetizing 

Z     7t 

force  of  the  fixed  coils  on  the  magnets.  Thus  if  we  know 
the  lead  of  the  brushes  and  the  current  in  the  armature  we 
are  at  once  in  a  position  to  calculate  the  effect  on  the 
electromotive  force  of  the  machine.  A  further  effect  of 
the  current  in  the  armature  is  a  material  disturbance  in 
the  distribution  of  the  induction  over  the  bored  face  of 
the  pole  piece;  the  force  along  B  C  (Fig.  34)  is  by  no 
means  equal  to  that  along  D  E.  Draw  the  closed  curve 
B  C  G  H B:  the  line  integral  along  C  G  and  H B  is  negli- 
gible. Hence  the  difference  between  force  H  G  and  B  C 

is  equal  to  4;r  (7  —  —  =  %Km  C,  where  K  is\the  angle  COG. 

This  disturbance  has  no  material  effect  upon  the  perform- 
ance of  the  machine.  Hut  the  current  in  the  armature 
also  distorts  the  arrangement  of  the  comparatively  weak 
field  in  the  gap  between  the  pole  pieces,  displacing  the 
point  of  zero  field  in  the  direction  of  rotation  in  a  gener- 
ator and  opposite  to  the  direction  of  rotation  in  a  motor; 
and  it  is  due  to  this  that  the  non-sparking  point  of 
the  brushes  is  displaced.  A  satisfactory  mathematical 
analysis  of  the  displacement  of  the  field  in  the  gap  be- 
tween the  pole  pieces  by  the  current  in  the  armature 
would  be  more  troublesome  than  an  a  priori  analysis  of 
the  distribution  of  field  in  this  space  when  the  magnet 
current  is  the  only  magnetizing  force.  Owing  to  the  fact 
that  the  armature  is  divided  into  a  finite  number  of  sec- 
tions, there  is  a  rapid  diminution  of  the  displacement  of 


106   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

the  field  during  the  time  that  a  section  is  being  commu- 
tated,  the  diminution  being  recovered  while  the  brush  is 
in  contact  with  only  one  bar  of  the  commutator.  The 
field  thus  oscillates  slightly,  owing  to  the  disturbance 
caused  by  reversing  the  direction  of  the  current  in  tin- 
successive  sections  of  the  armature.  The  number  of  oscil- 
lations in  a  Gramme  armature  or  in  a  Siemens  armature 
with  an  even  number  of  sections  will  be  p  m,  where  p  is 
the  number  of  revolutions  per  second;  but  in  a  Siemens 
armature  with  an  odd  number  of  sections  it  will  be  2p  m.* 
This  oscillation  of  the  field  is  only  another  way  of  express- 
ing the  effect  of  the  self  induction  of  the  section,  but  it 
must  be  remembered  that  if  the  self  induction,  multiplied 
by  change  of  current,  is  expressed  as  a  change  in  the  field 
we  must  omit  self  induction  as  a  separate  term  in  our 
electrical  equations.  The  precise  lead  to  be  given  to  the 
brushes  in  order  to  avoid  spa_rking  in  any  given  case 
depends  on  many  circumstances — the  form  and  extent  of 
the  pole  pieces,  the  number  of  sections  in  the  armature, 
and  the  duration  of  the  short  circuit  which  the  brushes 
cause  in  any  section  of  the  armature.  The  adjustment  of 
the  position  of  the  collecting  brushes  is  generally  made  by 

*  Added  Aug.  1?.— Armatures  with  an  odd  number  of  convolutions  are  open 
to  one  theoretical  objection,  which  would  be  a  practical  one  if  the  number  of 
convolutions  were  very  small  The  2m  •  1  convolutions  constitute  in  them- 
selves a  closed  circuit,  having  a  resistance  four  times  the  mean  actual  resist- 
ance  of  the  armature  measured  between  the  collecting  brushes.  When  any 
one  convolution  is  exactly  in  the  middle  of  the  field,  the  E.M.F.  of  the  other  2m 
convolutions  exactly  balance,  so  that  there  is  upon  the  closed  circuit  an  E.M.F. 

due  to  the  single  convolution  somewhat   in  excess  of  —  part  of  the  actual 

E.M.F.  of  the  ?nachine.  Thus  there  will  be  an  alternating  E.M.F.  around  the 
closed  circuit  of  the  armature  capable  of  causing  a  considerable  waste  of 
power.  This  waste  is  materially  checked  by  the  sHf  induction  of  the  circuit. 


DYNAMO-ELECTRIC   MACHINERY.  107 

hand  at  the  discretion  of  the  attendant,  and  is  in  some 
cases  fixed  once  for  all  to  suit  an  average  condition  of  the 
machine.  We  shall,  therefore,  treat  A.  the  lead  as  an  inde- 
pendent variable,  controlled  by  the  attendant. 

Let  /  be  total  induction  through  the  armature,  /-j-  1' 
total  induction  through  the  magnets,  /'  being  the  waste 
field.  Let  C  be  current  in  armature,  c  in  the  magnets. 
Let  g  1'  be  the  line  integral  of  magnetic  force  from  a  point 
on  one  pole  piece  to  a  point  on  the  other;  the  line  being 
drawn  external  to  the  armature,  g  will  be  approximately 
constant.  Omitting  as  comparatively  unimportant  the 
magnetizing  force  in  the  pole  pieces  and  iron  core  of  the 
armature,  we  have  the  following  equations  :— 

/ 

4  A,  m  C  -f  219  -r  —  g  I'  =  0; 

4\mC+2l,~ 
~t 

When  C  =  0,vf&  observed 
1  = 


r  —  1 
whence 

g=      *     8*. 

eliminating  /', 


—  4-Trnc 


108   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

The  characteristic  curve  when  C  =  0  being  7  =  F(±  nnc), 
we  may  write  the  above  as  the  equation  of  the  character- 
istic surface  thus: 

4XrnC 


In  applying  this  equation  it  must  not  be  forgotten  that 
the  E.  M.  F.  of  the  machine  cannot  be  determined  from  / 


Fio.  35. 


unless  the  commutation  occurs  at  such  a  time  that  the  coil 
being  commutated  embraces  all,  or  nearly  all,  the  lines  of 
induction  in  the  armature. 

This  equation  enables  the  characteristic  surface  to  be 


DYNAMO-ELECTRIC   MACHINERY.  109 

constructed  from  the  characteristic  curve.  Let  0  L  =  4  n  n  c 
(Fig.  35),  LM=±m\C;  draw  MK  so  that  j^=\\ 

through  ./Tdraw  ordinate  KR,  meeting  characteristic  curve 
in  R  ;  draw  R  Q  parallel  to  0  L,  meeting  ordinate  Q  L  in  Q  ; 

•p  c» 

draw    Q  S    parallel    to    L  M\     draw    Q  P    so    that 


.  ^y  .     Then  P  is  a  point  on  the  characteristic 


surface. 

A  very  important  problem  is  to  deduce  the  characteristic 
curve  of  a  series-  wound  machine  from  the  normal  charac- 
teristic; in  this  case  c  =  C,  and  we  have 


taking  PR  (Fig.  36)  as  ordinate  of  any  point  in  the  nor- 

v  —  \  A 

mal  characteristic,  cut  off  QR  equal  to  —    -4Afw(7— * 

"V  £t ' 

that  is,  draw  0  Q  so  that 

tan$0£=—     —±\mC^/±n(n  —  - — \C 

_v  —  IA,       Am 

r     2L  A  m 

*  nn 

v 

Then  P  Q  will  represent  the  induction  corresponding  to 
magnetizing  force  ±n\n—  ^j  G.     It  is  noteworthy  that  as 


110  DYNAMO   MACHINERY  AND   ALLIED   SUBJECTS. 

the  current  C,  and  therefore  0  R,  increases,  PQ,  the  induc- 
tion, will  attain  a  maximum  and  afterwards  diminish,  van- 
ish, and  become  negative.  That  in  series-wound  machines 
the  E.  M.  F.  has  a  maximum  value  has  been  many  times 
observed.  The  cause  lies  in  the  existence  of  a  waste  field 


Fio.  86. 

not  passing  through  the  armature,  and  in  the  saturation  of 
the  magnet  core. 

The  effect  of  the  current  in  the  armature  on  the  poten- 
tial between  the  brushes  of  any  machine  is  the  same  as 
that  of  an  addition  to  the  resistance  of  the  armature  pro- 
portional to  the  lead  of  the  brushes  and  to  the  ratio  of  the 
waste  field  to  the  total  field,  combined  with  that  of  taking 

the  main  current  —  times  round  the  magnets  in  direction 


DYNAMO-ELECTRIC   MACHINERY.  Ill 

opposite  to  the  current  c.  The  preceding  investigation  tells 
the  whole  story  of  a  dynamo  machine,  excepting  only  the 
relation  of  A  to  C  in  order  that  the  brushes  may  be  so 
placed  as  to  avoid  sparking.  The  only  constant  or  func- 
tion which  has  to  be  determined  experimentally  for  any 
particular  machine  is  v,  the  ratio  of  total  to  effective  field; 
all  the  rest  follows  from  the  configuration  of  the  iron  and 
the  known  properties  of  the  material. 

The  following  illustrations  of  the  effect  of  the  current 
in  the  armature  and  the  lead  of  the  brushes  are  interesting. 
In  both  cases  the  magnet  coils  are  supposed  to  be  entirely 
disconnected,  so  that  c  is  zero.  First,  let  A  be  negative, 
short  circuit  the  brushes,  and  drive  the  machine  at  a  cer- 
tain speed;  a  large  current  will  be  produced,  the  current  in 
the  armature  itself  forming  the  magnet.*  Second,  let  A, 
be  positive,  cause  a  current  to  pass  through  the  armature: 
the  armature  will  turn  in  the  positive  direction  and  will  act 
as  a  motor  capable  of  doing  work.  In  either  case,  partic- 
ularly the  former,  such  use  of  the  machine  would  not  be 
practical,  owing  to  violent  sparking  on  the  commutator. 
The  following  is  a  further  illustration  of  the  formula 
given  above :  If  we  could  put  up  with  the  sparking  which 

*  Added  Aug.  17.— This  experiment  was  tried  upon  a  dynamo  machine  of 
construction  generally  similar  to  that  shown  in  Figs.  30,  31,  and  32,  but  with  an 
armature  of  half  the  length  intended  in  normal  work  to  give  400  amperes,  50 
volts,  at  1,000  revolutions.  The  magnet  coils  were  disconnected,  and  the  termi- 
nals of  the  armature  were  connected  through  a  Siemens  electrodynamometer, 
and  the  machine  was  run  at  1,380  revolutions.  When  the  brushes  were  placed  in 
the  normal  position  (A  =  0)  the  current  due  to  residual  magnetism  was  52  am- 
peres. By  giving  the  brushes  a  small  positive  lead  the  current  was  reduced  to 
nearly  zero.  By  giving  the  brushes  a  small  negative  lead  a  current  of  over  234 
amperes,  the  maximum  measured  by  the  dynamometer,  was  obtained,  and  by 
varying  the  lead  it  was  easy  to  maintain  a  steady  current  of  any  desired 
amount. 


DYNAMO   MACHINERY  AND  ALLIED   SUBJECTS. 

would  ensue,  it  would  be  possible  to  make  A  negative  in  a 
generator  of  electricity,  and  thereby  obtain  by  the  reactions 
of  the  armature  itself  all  the  results  usually  obtained  by 
compound  winding. 

EFFICIENCY   EXPERIMENTS. 

Having  discussed  the  relations  subsisting  between  the 
configuration  of  the  magnetic  circuit  of  a  dynamo  machine 
and  the  induction  obtained  for  given  magnetizing  forces, 
and  having  compared  the  results  obtained  by  direct  calcu- 
lation with  the  results  of  actual  observation  on  a  partic- 
ular machine,  the  construction  of  which  we  have  described 
at  length,  it  appeared  of  importance  to  determine  the 
efficiency  of  the  machine  under  consideration  'as  a  con- 
verter of  energy,  when  used  either  as  a  generator  of  elec- 
tricity or  as  a  motor.  An  accurate  determination  of  the 
mechanical  power  transmitted  to  a  dynamo  by  a  driving 
belt,  or  of  the  power  given  by  a  motor,  presents  formidable 
experimental  difficulties.  Moreover,  if  the  mechanical 
power  absorbed  in  driving  the  dynamo  be  measured  di- 
rectly, any  error  in  measurement  will  involve  an  error  of 
the  same  magnitude  in  the  determination  of  the  efficiency. 
To  avoid  this  difficulty,  we  employed  the  following  device: 

Let  two  dynamos,  approximately  equal  in  dimensions 
and  power,  have  their  shafts  coupled  by  a  suitable  coup- 
ling, which  may  serve  also  as  a  driving  pulley ;  and  let  the 
electrical  connections  between  the  dynamos  be  made  so  that 
the  one  drives  the  other  as  a  motor.  If  the  combination 
be  driven  by  a  belt  passing  over  the  coupling  pulley,  the 
power  transmitted  by  the  belt  is  the  waste  in  the  two  dyn- 


DYNAMO-ELECTRIC   MACHINERY.  113 

amos  and  the  connections  between  them.  By  suitably 
varying  the  magnetic  field  of  one  of  the  dynamos,  the 
power  passing  between  the  two  machines  can  be  adjusted 
as  desired.  If,  then,  the  electrical  power  given  out  by  the 
generator  is  measured,  and  also  the  power  transmitted  by 
the  belt,  the  efficiency  of  the  combination  can  be  at  once 
determined.  By  this  arrangement  the  measurement,  which 
presents  experimental  difficulties,  viz.,  the  power  trans- 
mitted by  the  belt,  is  of  a  small  quantity.  Consequently 
even  a  considerable  error  in  the  determination  hasXbtfta. 
small  effect  on  the  ultimate  result.  On  the  other  h^Qd 
the  measurement  of  the  large  quantity  involved,  viz.,  the 
electrical  power  passing  between  the  two  machines,  can 
without  difficulty  be  made  with  great  accuracy. 

The  second  machine  was  similar  in  all  respects  to  that 
already  described,  and  each  is  intended  for  a  normal  out- 
put of  105  volts,  320  amperes,  at  a  speed  of  750  revolutions 
per  minute. 

The  power  transmitted  by  the  belt  was  measured  by  a 
dynamometer  of  the  Hefner-Alteneck  type,  the  general 
arrangement  being  as  shown  in  the  diagram,  Fig.  37.  A 
is  the  driving  pulley  of  the  engine,  B  the  driven  coupling 
of  the  dynamos;  /),  D  are  the  guide  pulleys  of  the  dyna- 
mometer, carried  on  a  double  frame  turning  about  the  ful- 
crum C,  and  supported  by  a  spiral  spring,  the  suspension 
of  which  can  be  varied  by  a  pair  of  differential  pulley 
blocks  attached  to  a  fixed  support  overhead.  When  a  read- 
ing is  made,  the  suspension  of  the  spring  is  adjusted  until 
the  index  of  the  dynamometer  comes  to  a  fiducial  mark  on 
a  fixed  scale;  the  extension  of  the  spring  is  then  read  by  a 
second  index  attached  to  its  upper  extremity.  F9  F  are  two 


114  DYNAMO  MACHINERY  AND   ALLIED  SUBJECTS. 

fixed  guide  pulleys  of  the  same  diameter  as  the  pulleys  D, 
D,  and  having  the  same  distance  between  their  centres,  in 
order  that  the  two  portions  of  the  belt  may  be  parallel  and 
the  sag  as  far  as  possible  taken  up.  The  normal  from  G 


Fio.  87. 

to  the  centre  line  of  either  portion  of  the  belt  between  the 
pulley  B  and  the  guide  pulleys  =  31.9  cms.  The  normal 
from  C  to  the  centre  line  of  either  of  the  parallel  por- 
tions of  the  belt  =  2.4  cms.,  and  from  C  to  the  centre  line 
of  the  spring  =  92.7  cms. 

Take  moments  about  (7;  then 

92.7 

Tension  of  the  belt  =    j"0  X  tension  of  spring, 
o4.o 

=  2.7   X  tension  of  spring. 

Also  the  diameter  of  the  pulley  B  =  33.6  cms.  and  the 
thickness  of  the  belt  =  1.6  cm. 

Hence  the  velocity  of  the  centre  of  the  belt  in  centime- 


DYNAMO-ELECTKIC   MACHINERY.  115 

tres  per  second  =  1.845  X  revolutions  of  dynamo  per  minute, 
and,  therefore, 

Power  transmitted  by  the  belt  in  ergs  per  second  =  2.7  X 
1.845  X  981  X  tension  of  spring  X  revolutions  per  minute, 
assuming  the  value  of  g  to  be  981. 

We  may  more  conveniently  express  the  power  in  watts 
(=  107  ergs  per  second),  and  write 

Power  in  watts  =  0.0004887  X  tension  of  spring 

X  revolutions  per  minute. 

The  potential  between  the  terminals  of  the  generator 
was  measured  by  one  of  Sir  William  Thomson's  graded 
galvanometers,  previously  standardized  by  a  Clark's  cell, 
which  had  been  compared  with  other  Clark's  cells,  of  which 
the  electromotive  force  was  known  by  comparison  with 
Lord  Rayleigh's  standard.  The  current  between  the  two 
machines  was  measured  by  passing  it  through  a  known  re- 
sistance, the  difference  of  potential  between  the  ends  of  the 
resistance  being  determined  by  direct  comparison  with  the 
Clark's  standard  cell,  according  to  Poggendorff's  method. 
As  experiments  were  made  with  currents  of  large  magni- 
tude, it  was  important  that  the  temperature  coefficient  of 
the  resistance  should  be  as  low  as  possible.  To  this  end 
we  found  a  resistance  frame  constructed  of  platinoid  wire 
of  great  value.  The  temperature  coefficient  of  this  alloy 
is  only  0.021  per  cent,  per  degree  Centigrade.  (Proc.  Roy. 
Soc.,  vol.  xxxviii,  1885,  p.  265.) 

The  resistances  of  the  armatures  and  magnets  of  the 
two  machines  are  as  follows : — 


110  DYNAMO  MACHINERY   AND  ALLIED  SUBJECTS. 

Ohms. 
Generator,    .     .     .     armature,    .     .     .    0.009947 

magnets,      .     .     .  16.93 
Motor,     ....    armature,    .     .     .     0.009947 

magnets,      .     .     .  16.44 

The  resistance  of  the  leads  connecting  the  two  machines 
was  0.00205  ohm,  and  of  the  standard  resistance  0.00586 
ohm. 

In  all  determinations  of  resistance  the  value  of  the  B.  A. 


TERMINALS  Of  MACHINE 
ACTINGAS  MOTOR 


Fio.  88. 

ohm  was  taken  as  0.9867  X  10'  C.  G.  S.  units,  according  to 
Lord  Rayleigh's  determination. 

The  diagram,  Fig.  38,  shows  the  electrical  connections 
between  the  two  machines  with  the  rheostat  r  inserted  in 
the  magnets  of  the  motor  dynamo. 


DYNAMO- ELECTRIC   MACHINERY.  117 

In  order  to  ascertain  the  friction  of  bending  the  belt 
round  the  pulley  B,  and  of  the  journals  of  the  dynamo,  a 
preliminary  experiment  was  made  with  the  dynamometer. 
The  combination  was  run  at  a  speed  of  814  revolutions  per 
minute  with  the  dynamos  on  open  circuit,  and  the  tension 
of  the  spring  observed — 9,979  grams.  The  engine  was  then 
reversed  and  the  dynamos  run  at  the  same  speed,  and  the 
tension  of  the  spring  again  observed — 3,629  grams.  The 
difference  of  the  two  readings  gives  twice  the  power  ab- 
sorbed in  friction,  viz.,  1,262  watts  for  the  two  machines, 
or  631  watts  per  machine.  This  is  excluded  entirely  from 
the  subsequent  determinations  of  efficiency,  as  being  a 
quantity  dependent  on  such  arbitrary  conditions  as  the 
lubrication  of  the  journals,  the  weight  of  the  belt,  and  the 
angle  it  makes  with  the  horizontal. 

In  Table  V.,  column  I.  is  the  speed  of  the  dynamos; 
column  II.  is  the  reading  of  the  spring  in  grams;  column 

III.  is  the  power  transmitted  by  the  belt  in  watts;  column 

IV.  is   the  potential  at  the  terminals  of  the  generator; 
column  V.  is  the  current  passing  in  the  external  circuit  be- 
tween the  two  machines;  column  VI.  is  the  resistance  in- 
troduced into  the  magnets  of  the  motor  by  the  rheostat; 
column  VII.  is  the  power  absorbed  in  the  armature  of  the 
generator;    column  VIII.  is  the   power  absorbed  in  the 
armature  of  the  motor;  column  IX.  is  the  power  absorbed 
in  the  magnets  of  the  generator;  column  X.  is  the  power 
absorbed  in  the  magnets  of  the  motor;  column  XI.  is  the 
power  absorbed  in  the  connecting  leads  between  the  two 
dynamos,  in  the  rheostat  resistance  r,  and  in  the  standard 
resistance  used  for  measuring  the  current;  column  XII.  is 
the  total  electrical  power  developed  in  the  generator;  col- 


118   DYNAMO  MACHINERY   AND   ALLIED   SUBJECTS. 

TABLE  V. 


I. 

II. 

III. 

IV. 

V. 

VI. 

vn. 

Revolutions 

per 

Grams. 

Watts. 

Volts. 

Amperes. 

Ohms. 

Watts. 

minute. 

1 

810 

8,392 

3,322 

129.1 

21.6 

1.39 

13 

* 

801 

9,299 

3,640 

127.2 

72.0 

1.89 

75 

3 

811 

11,113 

«,»"••> 

125.8 

150.0 

2.72 

267 

4 

808 

10,433 

4,119 

124.4 

186.0 

2.72 

8'.>7 

5 

792 

10,000 

4,124 

116.5 

211.0 

2.72 

4«y 

6 

798 

ltt,S97 

t;.:,y. 

110.6 

351.0 

4.59 

1,309 

7 

764 

17,0<JO 

.;..,,»:, 

110.12 

358.0 

4  c'.i 

1,300 

8 

706 

17,804 

6,065 

110.6 

360.0 

4.59 

1,875 

9 

778 

16,556 

6,294 

102  3 

369.0 

4.09 

1,430 

10 

756 

20,412 

7,541 

90.8 

i  »»;  o 

4.59 

2,070 

11 

NIS 

9,52(5 

3,765 

119.3 

36.8 

2.72 

25 

12 

802 

3,855 

1,512 

113.5 

No  current 

.... 

13 

814 

8,175 

1,262 

.... 

.... 

.... 

VIII. 
Watts. 

IX. 

Watts. 

X. 

Watts. 

XI. 
Watts. 

XII. 
Watts. 

XIII. 
Watts. 

XIV. 
Watte. 

1 

4 

984 

861 

77 

4,?a) 

691 

5,411 

2 

51 

955 

837 

112 

11.096 

805 

11,901 

3 

223 

985 

709 

295 

20,8% 

9K8 

21>S| 

4 

344 

914 

BBS 

388 

85,266 

691 

2.v.»»y 

5 

443 

801 

608 

453 

26..V.K) 

BOO 

gr,tto 

6 

1,222 

722 

455 

1,101 

41.433 

no 

42,323 

7 

1,268 

716 

473 

1.131 

42.087 

828 

42.915 

8 

1  289 

722 

455 

1,152 

42,  194 

Kili 

18,880 

9 

1  354 

618 

408 

1,178 

40,314 

SAO 

40.984 

10 

1979 

554 

848 

1,670 

46.244 

459 

M.7M 

11 

13 

841 

637 

116 

5,998 

1,006 

7,064 

12 



756 

13 

... 



.... 

631 



umn  XIII.  is  half  the  power  absorbed  by  the  combination 
less  the  known  losses  in  the  armatures,  magnets,  and  ex- 
ternal connections  of  the  two  machines;  column  XIV.  is 
the  total  mechanical  power  given  to  the  generator,  being 
the  sum  of  the  powers  given  in  columns  XII.  and  XIII. 

In  Table  VI.  the  percentage  losses  in  the  armature  and 
magnets  of  the  generator  are  given,  as  also  the  sum  of  all 


DYNAMO-ELECTRIC    MACHINERY. 
TABLE  VI. 


119 


I. 

II. 

III. 

IV. 

V. 

VI. 

Per  cent. 

Per  cent. 

Per  cent 

Per  cent. 

Per  cent. 

Per  cent. 

1 

0.24 

18.20 

12.76 

68.8 

57.28 

39.40 

2 

0.63 

8.93 

6.76 

84.58 

82.99 

70.19 

3 

1.88 

4.27 

4.52 

90.00 

90.15 

81  .  13 

4 

1.53 

3  52    . 

2.66 

92.28 

92.65 

85.49 

5 

1.83 

2.94 

2.415 

92.80 

93.12 

86.42 

6 

3.09 

1.71 

2.10 

93.10 

93.30 

86.86 

7 

3.17 

1.67 

1.93 

93  23 

93.39 

87.07 

8 

3.17 

1.67 

1.93 

93.23 

93.43 

87.10 

9 

3.51 

1.75 

1.59 

93.39 

93.50 

87.32 

10 

4.43 

1.19 

0.98 

93.39 

93.36 

87.19 

11 

0.35 

11.9 

15.1 

72.65 

65.77 

47.78 

other  losses  as  obtained  from  column  XIII.  in  Table  V.; 
also  the  percentage  efficiency  of  the  generator,  of  the  motor, 
and  of  the  double  conversion.  Column  I.  is  the  percentage 
loss  in  the  generator  armature;  column  II.  is  the  percent- 
age loss  in  the  generator  magnets;  column  III.  is  the  per- 
centage sum  of  all  other  losses  in  the  generator;  column 

IV.  is  the  percentage  efficiency  of  the  generator;  column 

V.  is  the  percentage  efficiency  of  the  motor;  column  VI. 
is  the  percentage  efficiency  of  the  double  conversion. 

In  this  series  of  experiments,  in  all  cases  from  Nos.  1  to 
10  inclusive,  the  brushes,  both  of  the  generator  and  motor, 
were  set  at  the  non-sparking  point;  but  in  No.  11  no  lead 
was  given  to  the  brushes  of  the  generator,  and  consequently 
there  was  violent  sparking  throughout  the  duration  of  the 
experiment. 

In  No.  12  the  magnets  were  separately  excited  with  a 
current  giving  113.5  volts  across  their  terminals.  The 
power  absorbed  must  be  due  entirely  to  local  currents  in 
the  core  of  the  armature  and  to  the  energy  for  the  reversal 
of  magnetization  of  the  core  twice  in  every  revolution  of 
the  armature. 


120  DYNAMO   MACHINERY    AND   ALLIED   SUBJECTS. 

No.  13  gives  the  results  of  the  experiments  on  the  fric- 
tion of  the  bearings  and  in  bending  the  belt  already 
referred  to. 

It  will  be  observed  that  the  figures  in  column  XIII.  are 
calculated  by  deducting  the  power  absorbed  in  the  arma- 
tures and  magnets  and  by  extraneous  resistances  from  the 
total  power  given  to  the  combination  as  measured  by  the 
dynamometer.  They  must  therefore  include  all  the  energy 
dissipated  in  the  core  of  the  armature,  whether  in  local 
currents  or  in  the  reversal  of  its  magnetization ;  also  the 
energy  dissipated  in  local  currents  in  the  pole  pieces,  if 
such  exist;  also  the  energy  spent  in  reversing  the  direction 
of  the  current  in  each  convolution  of  the  armature  as  they 
are  successively  short  circuited  by  the  brushes.  Further, 
it  will  include  the  waste  in  all  the  connections  of  the  ma- 
chine from  the  commutator  to  its  terminals  and  the  friction 
of  the  brushes  against  the  commutator.  A  separate  experi- 
ment was  made  to  determine  the  amount  of  this  last 
constituent,  but  it  was  found  to  be  too  small  to  be  capable 
of  direct  measurement  by  the  dynamometer.  Moreover, 
from  the  manner  in  which  the  figures  in  this  column  are 
deduced,  any  error  in  the  dynamometric  measurement  will 
appear  wholly  in  them.  Since,  undoubtedly,  the  first  two 
components  enumerated  are  the  most  important,  and  the 
conditions  determining  their  amount  are  practically  the 
same  throughout  the  series,  the  close  agreement  of  the 
figures  in  the  column  is  a  fair  criterion  of  the  accuracy 
of  the  observations.  Probably  100  watts  is  the  limit  of 
error  in  any  of  the  measurements.  Such  an  error  would 
affect  the  determination  of  the  efficiency  when  the  machines 
were  working  up  to  their  full  power  by  less  than  £  per  cent. 

It  has  been  assumed  that  the  sum  of  these  losses  is 


DYNAMO-ELECTRIC   MACHINERY. 

equally  divided  between  the  two  machines.  This  will  not 
accurately  represent  the  facts,  as  the  intensities  of  the 
fields  and  the  currents  passing  through  the  armatures 
differ  to  some  extent  in  the  two  machines.  The  inequality, 
however,  cannot  amount  to  a  great  quantity,  and  if  it 
diminishes  the  efficiency  of  the  generator  it  will  increase 
the  efficiency  of  the  motor  by  a  like  amount,  and  contrari- 
wise. In  No.  11  of  the  series  the  effect  of  the  sparking  at 
the  brushes  of  the  generator  is  very  marked,  the  power 
wasted  amounting  to  at  least  250  watts. 

If  it  be  assumed  that  the  dissipation  of  energy  is  the 
same  whether  the  magnetization  of  the  core  is  reversed  by 
diminishing  and  increasing  the  intensity  of  magnetization 
without  altering  its  direction,  or  whether  it  is  reversed  by 
turning  round  its  direction  without  reducing  its  amount 
to  zero,  a  direct  approximation  may  be  made  to  the  value  of 
this  component.  (J.  Hopkinson,  Phil.  Trans.,  vol.  clxxvi, 
1885,  p.  455.) 

The  core  has  about  16,400  cubic  centimetres  of  soft  iron 
plates;  hence  loss  in  magnetizing  and  demagnetizing  when 
the  speed  is  800  revolutions  per  minute  =  16,400  X  *f£-  X 
13,356  ergs  per  second  =  292  watts. 

Eeferring  to  Table  VI.,  it  appears  that  the  efficiency  ap- 
proaches a  maximum  when  the  current,  passing  externally 
between  the  two  machines,  is  about  400  amperes.  Let  C 
be  the  current  in  the  armature,  p  its  resistance,  W  the 
power  absorbed  in  all  parts  of  the  machine  other  than  the 
armature;  then,  if  the  speed  is  constant,  the  efficiency  is 

.  EC -W-(?p       .  _   .      ,, 

approximately  -    — =-~ — ,   where    E  is    the    electro- 

&  L> 

W 

motive  force,    This  is  a  maximum  when   7*  +  C p  is  a 

0 


122   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

minimum,  which  occurs  when  W=  C*p;  when  the  loss  in 
the  armature  is  equal  to  the  sum  of  all  other  losses.  For 
the  machines  under  consideration  the  experimental  results 
verify  this  deduction.  But  in  actual  practice  the  rate  of 
generation  of  heat  in  the  armature  conductors,  when  a 
current  of  400  amperes  was  passed  for  a  long  period,  would 
be  so  great  as  to  trench  upon  the  margin  of  safety  de- 
sirable in  such  machines.  Of  the  total  space,  however, 
available  for  the  disposition  of  the  conductors,  only  about 
one  fourth  part  is  actually  occupied  by  copper,  the  re- 
mainder being  taken  up  with  insulation  and  the  inter- 
stices left  by  the  round  wire.  If  the  space  occupied  by 
the  copper  should  be  increased  to  three  fourths  of  the  total 
space  available,  while  the  cooling  surface  remained  the 
same,  the  current  could  be  increased  75  per  cent,  and  the 
efficiency  increased  1.3  per  cent,  approximately,  as  all 
losses  other  than  that  in  the  armature  wires  would  not  be 
materially  altered. 

The  loss  in  the  magnets  is  also  susceptible  of  reduction. 
It  has  already  been  shown  that  for  a  given  configuration 
of  the  magnetic  circuit  and  a  given  electromotive  force  the 
section  of  the  wire  of  the  magnet  coils  is  determinate. 
The  length  is,  however,  arbitrary,  since  within  limits  the 
number  of  ampere  convolutions  is  independent  of  the 
length.  An  increase  in  the  length  will  cause  a  propor- 
tionate diminution  in  the  power  absorbed  in  the  magnet 
coils.  If  the  surface  of  the  magnets  is  sufficient  to  dissi- 
pate all  the  heat  generated,  then  the  length'  of  wire  is 
properly  determined  by  Sir  William  Thomson's  rule  that 
the  cost  of  the  energy  absorbed  must  be  equal  to  the  con- 
tinuing cost  of  the  conductor. 


DYNAMO-ELECTRIC  MACHINERY.  123 

APPENDIX. 
(Added  Aug.  17.) 

Since  the  reading  of  the  present  communication  experi- 
ments have  been  tried  on  machines  having  armatures 
wound  on  the  plan  of  Gramme  and  with  differently  ar- 
ranged magnets;  the  experiments  were  carried  out  in  a 
closely  similar  manner  to  that  already  described. 

DESCRIPTION  OF  MACHINES. 

The  construction  of  these  machines  is  shown  in  Figs.  39, 


Fio.  39. 


40,  and  41,  of  which  Fig.  39  shows  an  elevation,  Fig.  40  a 
section  through  the  magnets,  Fig.  41  a  longitudinal  sec- 
tion of  the  armature.  It  will  be  observed  that  the  magnetic 
circuit  is  divided.  The  pole  pieces  are  of  cast  iron  and 


124  DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

are  placed  above  and  below  the  armature  and  are  extended 
laterally.  The  magnet  cores  are  of  wrought  iron  of  cir- 
cular section  and  fit  into  the  extensions  of  the  east  iron 


Fio.  40. 


pole  pieces,  so  that  the  area  of  contact  of  the  cast  iron  is 
greater  than  the  area  of  section  of  the  magnet.  The  mag- 
netizing coils  consist  of  2,196  convolutions  on  each  limb 


Fio.  41. 


of  copper  wire,  No.  17,  B.W.G.,  in  No.  1  machine,  and 
2,232  convolutions  in  No.  2  machine.  The  pole  pieces  are 
bored  to  receive  the  armature,  leaving  a  gap  on  either  side 
subtending  an  angle  of  41°  at  the  axis. 


MACIUNEkY.  125 

The  bearings  are  carried  upon  an  extension  of  the  lower 
pole  piece. 

The  following  table  gives  the  principal  dimensions  of 
the  magnets  in  No.  1  machine : — 

cms. 

Length  of  magnet  limbs  between  pole  pieces 26.0 

Diameter  of  magnet,  limb 15.24 

Boreof  fields 26.7 

Width  of  pole  piece  parallel  to  the  shaft 24.1 

Width  of  gap  between  poles 8.6 

The  armature  is  built  up  of  plates  as  in  the  machine  al- 
ready described,  and  is  carried  from  the  shaft  by  a  brass 
frame  between  the  arms  of  which  the  wires  pass. 

The  principal  dimensions  are  as  follows: — 

cms. 

Diameter  of  core 24.1 

Diameter  of  hole  through  core I  14.0 

Length  of  core  over  end  plates 24.1 

The  core  is  wound  on  Gramme's  principle  with  160  con- 
volutions, each  consisting  of  a  single  wire,  No.  9,  B.W.G., 
the  wire  lying  on  the  outside  of  the  armature  in  a  single 
layer.  The  commutator  has  40  bars. 

This  dynamo  is  compound  wound,  and  is  intended  for  a 
normal  output  of  105  volts,  130  amperes,  at  a  speed  of 
1,050  revolutions  per  minute.  The  resistance  of  the 
armature  is  0.047  ohm,  and  of  the  magnet  shunt  coils  26.87 
ohms. 

There  is  here  no  yoke,  and  consequently  At  and  lt  do  not 
appear  in  the  equation. 

It  is  necessary  to  bear  in  mind  that  the  magnetizing 
force  is  that  due  to  the  convolutions  on  one  limb,  and  that 
the  areas  are  the  sums  of  the  areas  of  the  two  limbs.  In  cal- 


126  DYNAMO   MACHINERY    AND   ALLIED   SUBJECTS. 

dilating  induction  from  E.M.F.  it  is  also  necessary  to  remem- 
ber that  two  convolutions  in  a  Gramme  count  as  one  in  a 
Hefner-Alteneck  armature. 

A^y — the  section  of  the  core  is  245  sq.  cms.;  allowances 
for  insulation  reduce  this  to  220.5  sq.  cms. 

/, ; — this  is  assumed  to  be  10  cms.,  but  it  will  be  seen 
that  an  error  in  this  value  has  a  much  more  marked 
effect  on  the  characteristic  in  this  machine  than  in  the 
other. 

At; — the  angle  subtended  by  the  bored  face  of  the 
pole  pieces  is  139°;  the  mean  of  the  radii  of  the  pole  pieces 
and  the  core  is  12.45  cms.  Hence  the  area  of  139°  of 
the  cylinder  of  this  radius  is  768.3  sq.  cms. ;  add  to  this  a 
fringe  of  a  width  0.8  of  the  distance  from  core  to 
pole  pieces,  as  already  found  necessary  for  the  other 
machine,  and  we  have  839.5  sq.  cms.  as  the  value  of  At. 

J,  is  0.8  cm. 

At  is  365  sq.  cms.  (i.e.,  the  area  of  two  magnet  cores). 

lt  is  26.0  cms. 

At  is  taken  to  be  532  sq.  cms.,  viz.,  double  the  smallest 
section  of  the  pole  piece. 

7,  is  a  very  uncertain  quantity;  it  is  assumed  to  be  15 
cms. 

The  expression  already  used  requires  slight  modification. 
Inasmuch  as  the  pole  pieces  are  of  cast  iron,  a  different 
function  must  be  used.  Different  constants  for  waste  field 
must  be  used  for  the  field,  the  pole  pieces,  and  the  magnet 
core.  We  write 


DYNAMO-ELECTRIC   MACHINERY.  127 

The  function  f  is  taken  from  Hopkinson,  Phil.  Trans.,  vol. 
clxxvi,  1885,  p.  455,  Plate  52.  v^  vw  and  vb  were  deter- 
mined by  experiment,  as  described  below;  their  values  are 

vt  =  1.05 
v,  =  1.18 
rb  =  1.49 

Comparing  the  curves  in  Figs.  28  and  29  with  that  in 
Figs.  42  and  43,  the  most  notable  difference  is  that  in  the 
present  case  the  armature  core  is  more  intensely  magnetized 
than  the  magnet  cores.  No  published  experiments  exist 
giving  the  magnetizing  force  required  to  produce  the  in- 
duction here  observed  in  the  armature  core,  amounting  to 
a  maximum  of  20,000  per  sq.  cm.  We  might,  however, 
make  use  of  such  experiments  as  the  present  to  construct 
roughly  the  curve  of  magnetization  of  the  material;  thus 
we  find  that  with  this  particular  sample  of  iron  a  force  of 
740  per  cm.  is  required  to  produce  induction  20,000  per  sq. 
cm. :  this  conclusion  must  be  regarded  as  liable  to  consider- 
able uncertainty. 

The  observations  on  the  two  machines  are  plotted  to- 
gether, but  are  distinguished  from  each  other  as  indicated. 
They  are,  unfortunately,  less  accurate  than  those  of  Figs.  28 
and  29,  and  are  given  here  merely  as  illustrating  the  method 
of  synthesis. 


EXPERIMENTS  TO   DETERMINE 

The  method  was  essentially  the  same  as  is  described  on 
pp.  96   to   99,  and  was  only  applied  to   No.   1  machine. 


128   DYNAMO  MACHINERY  AND   ALLIED  SUBJECTS. 


s 

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DYNAMO-ELECTKIO  MACHINERY.  129 


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on  in  10 

In 

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„  • 

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fbrce 

FIG.  43.— SYNTHESIS  OF  CHARACTERISTIC  CURVE  WITH  GRAMME  ARMATURE. 
This  figure  is  the  same  as  the  left-hand  part  of  Fig.  42,  but  on  a  larger  scale. 


130  DYNAMO   MACHINERY  AND   ALLIED   SUBJECTS. 

Keferring  to  Fig.  44,  a  wire  A  A  was  taken  four  times  round 
the  middle  of  one  limb  of  the  magnet,  a  known  current  was 
suddenly  passed  round  the  magnets,  and  the  elongation  of 
the  reflecting  galvanometer  was  observed :  it  was  found  to 
be  214  scale  divisions,  giving  107  as  the  induction  through 


Fio.  44. 


the  two  magnet  limbs  in  terms  of  an  arbitrary  unit.  The 
coil  was  moved  to  the  top  of  the  limb  as  at  B  B ;  the  elon- 
gation was  reduced  to  206,  or  103  for  the  two  limbs;  we 
take  the  mean  induction  in  the  magnet  to  be  105.  A  wire 
was  taken  three  times  round  the  whole  armature  in  a  hori- 
zontal plane  as  at  C  C\  the  elongation  observed  was  222 
divisions  or  74  in  terms  of  the  same  units.  A  wire  was 
taken  four  times  round  one  half  of  the  armature  as  at  D  D\ 


DYNAMO-ELECTRIC   MACHINERY.  131 

the  elongation  was  141,  or  induction   in   the  iron  of  the 
armature  70.5,  whence  we  have 

74     :=1.05. 


70.5 

It  may  be  well  to  recall  here  that  rt  is  essentially  depend- 
ent on  the  intensity  of  the  field;  strictly  the  line  B  in 
Figs.  42  and  43  should  not  be  straight,  but  slightly  curved. 

Four  coils  were  taken  round  the  upper  pole  piece  at  E  E\ 
the  elongation  was  159,  giving  79.5  on  the  two  sides.  Coils 
at  F  F  give  a  higher  result,  87.5,  owing  to  the  lines  of  in- 
duction which  pass  round  by  the  bearings  of  the  machine, 
and  across  to  the  upper  ends  of  the  magnets.  v6  is  taken 

to  be          =  1.18. 


EFFICIENCY   EXPERIMENTS. 

The  method  and  instruments  were  those  already  de- 
scribed, pp.  110  to  112,  excepting  that  the  current  was 
measured  by  a  Thomson's  graded  galvanometer,  which  had 
been  standardized  against  a  Clark's  cell  in  the  position  and 
at  the  time  when  used.  The  resistance  of  leading  wires 
and  galvanometer  was  0.034  ohm,  the  series  coils  introduced 
for  compounding  the  machines  were  also  brought  into  use, 
and  the  losses  due  to  their  resistance  (0.024  ohm)  find  a  place 
in  columns  XII.  and  XIII.  of  Table  VII.,  in  which  column 
I.  is  lead  of  brushes  of  the  dynamo,  positive  for  the  gene- 
rator, negative  for  the  motor;  column  II.,  revolutions  per 
minute;  column  III.,  deflection  of  spring  in  grams;  column 


132  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


TABLE  VII. 


I. 

n. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

Degrees. 

Revolutions. 

Grains. 

Watts. 

Volts. 

Amperes 

Ohms. 

Watts. 

17.5 

1098 

7711 

4419 

100.1 

139.0 

00 

955 

5 

1094 

2722 

1554 

103.8 

41.2 

18.8 

10:. 

0 

1114 

1814 

1063 

104.7 

7.85 

0 

11 

IX. 

X. 

XI. 

XII. 

xm. 

XIV. 

XV. 

XVI. 

XVII. 

Watts. 

Watts. 

Watts. 

Watts. 

Watts. 

Watts. 

Watts. 

Watts. 

Watts. 

895 

872 

0 

497 

464 

657 

16,395 

289 

16,684 

78 

400 

138 

5ft 

41 

146 

5.015 

294 

r,.:xi 

3 

408 

406 

6 

1 

2 

1,687 

128 

1,765 

*  In  this  experiment  the  direction  of  the  current  had  become  reversed,  and 
No.  2  machine  was  generator. 

IV.,  watts  by  dynamometer;  column  V.,  volts  at  terminals  of 
generator;  column  VI.,  amperes  in  external  circuit;  column 
VII.,  rheostat  resistance;  column  VIII.,  watts  in  generator 
armature;  column  IX.,  watts  in  motor  armature;  column 
X.,  watts  in  generator  shunt  magnet  coils;  column  XL, 
watts  in  motor  shunt;  column  XII.,  watts  in  generator 
series  magnet  coils;  column  XIII.,  watts  in  motor  series; 
column  XIV.,  watts  in  external  resistances;  column  XV., 
total  electrical  power  of  generator;  column  XVI.,  half  the 
sum  of  losses  unaccounted  for;  column  XVII.,  total  me- 
chanical power  applied  to  generator. 

TABLE  VIII. 


Generator  Generator 
Armature;    Shunt 

Generator 
Series 
Coils. 

Other 
Losses. 

Efficiency 
of 
Generator. 

Efficiency 
of 
Motor. 

Efficiency 
of  Double 
Conversion. 

5.8 
2.0 

2.2 
7.5 

3.0 
1.0 

1.9 
5.5 

87.1 
84.0 

89.0 
92.0 

77.5 
77.8 

DYNAMO-ELECTRIC   MACHINERY.  133 

Table  VIII.  gives  the  losses  and  efficiencies  as  percent- 
ages in  exactly  the  same  way  as  in  Table  VI.,  excepting 
that  another  column  is  introduced  for  the  loss  in  the  series 
coils  of  the  magnets  of  the  generator. 

The  core  of  the  armature  contains  about  6,500  cub.  cms. 
of  iron.  Hence  energy  of  magnetizing  and  demagnetizing 
when  the  speed  =  1,100  revolutions  per  minute  =  6,500  X 

'—  X  13,356  in  ergs  per  second  =  159  watts. 


134  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


DYNAMO-ELECTRIC   MACHINERY.* 

THE  following  is  intended  as  the  completion  of  a  Paperf 
by  Drs.  J.  and  E.  Hopkinson  (Phil  Trans.,  1886). f  The 
motive  is  to  verify  by  experiment  theoretical  results  con- 
cerning the  effect  of  the  currents  in  the  armature  of  dyna- 
mo machines  on  the  amount  and  distribution  of  the  mag- 
netic field  which  were  given  in  that  Paper,  but  which  were 
left  without  verification.  For  the  sake  of  completeness, 
part  of  the  work  is  given  over  again. 

The  two  dynamos  experimented  upon  were  constructed 
by  Messrs.  Siemens  Brothers  &  Co.,  and  are  identical  as 
far  as  it  is  possible  to  make  them.  They  are  mounted 
upon  a  common  base  plate,  their  axles  being  coupled  to- 
gether, and  are  referred  to  in  this  Paper  respectively  as 
No.  1  and  No.  2. 

Each  dynamo  has  a  single  magnetic  circuit  consisting  of 
two  vertical  limbs  extended  at  their  lower  extremities  to 
form  the  pole  pieces,  and  having  their  upper  extremities 
connected  by  a  yoke  of  rectangular  section.  Each  limb, 

*  It  must  not  be  supposed  from  his  name  not  appearing  in  this  short  Paper 
that  my  brother.  Dr.  E.  Hopkinson.  had  a  minor  part  in  the  earlier  Paper.  He 
not  only  did  the  most  laborious  part  of  the  experimental  work,  but  contributed 
his  proper  share  to  whatever  there  may  be  of  merit  in  the  theoretical  part  of 
the  Paper.— J.  H. 

t  The  Paper  here  referred  to  is  that  reprinted  on  pages  79  to  133  of  this 
volume. 


DYNAMO-ELECTRIC   MACHINERY.  135 

together  with  its  pole  piece,  is  formed  of  a  single  forging 
of  wrought  iron.  These  forgings,  as  also  that  of  the  yoke, 
are  built  up  of  hammered  scrap  iron,  and  afterwards  care- 
fully annealed.  Gun-metal  castings  bolted  to  the  base 
plate  of  the  machine  support  the  magnets. 

The  magnetizing  coils  on  each  limb  consist  of  sixteen 
layers  of  copper  wire  2  mms.  in  diameter,  making  a  total  of 
3,968  convolutions  for  each  machine.  The  pole  pieces  are 
bored  out  to  receive  the  armature,  leaving  a  gap  above  and 
below  subtending  an  angle  of  68°  at  the  centre  of  the  shaft. 
The  opposing  surfaces  of  the  gap  are  1.4  cm.  deep. 

The  following  table  gives  the  leading  dimensions  of  the 
machine: — 

cms. 

Length  of  magnet  limb 66.04 

Width  of  inaguet  limb  11.48 

Breadth  of  magnet  limb 38.10 

Length  ofyoke  38.10 

Width  of  yoke 12.06 

Depth  ofyoke 11.43 

Distance  between  centres  of  limbs 23.50 

Bore  of  fields 21.21 

Depth  of  pole  piece 20.32 

Thickness  of  gun-metal  base 10.80 

Width  of  gap 12.06 

The  armature  core  is  built  up  of  soft  iron  disks,  No.  24 
B.  W.  G.,  which  are  held  between  two  end  plates  screwed  on 
the  shaft. 

The  following  table  gives  the  leading  dimensions  of  the 
armature : — 

cms. 

Diameter  of  core 18.41 

Diameter  of  shaft. 4.76 

Length  of  core , 38,10 


136   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

The  core  is  wound  longitudinally  according  to  the  Hef- 
ner von  Alteneck  principle  with  208  bars  made  of  copper 
strip,  each  9  mms.  deep  by  1.8  mm.  thick.  The  commuta- 
tor is  formed  of  fifty-two  hard  drawn  copper  segments  in- 
sulated with  mica,  and  the  connections  to  the  armature  so 
made  that  the  plane  of  commutation  in  the  commutator  is 
vertical  when  no  current  is  passing  through  the  armature. 

Each  dynamo  is  intended  for  a  normal  output  of  80  am- 
peres, 140  volts,  at  880  revolutions  per  minute.  The  resist- 
ance of  the  armature  measured  between  opposite  bars  of 
the  commutator  is  0.042  ohm,  and  of  each  magnet  coil 
13.3  ohms. 

In  the  machine  the  armature  core  has  a  greater  cross 
section  than  the  magnet  cores,  and  consequently  the  mag- 
netizing force  used  therein  may  be  neglected.  The  yoke 
has  the  same  section  as  the  magnet  cores,  and  is  therefore 
included  therein,  as  is  also  the  pole  piece.  The  formula 
connecting  the  line  integral  of  the  magnetizing  force  and 
the  induction  takes  the  short  form 


where 

n  is  the  number  of  turns  round  magnet; 

c  is  the  current  round  magnet  in  absolute  measure; 

/Q  the  distance  from  iron  of  armature  to  rim  of  magnet; 

AI  the  corrected  area  of  field; 

/the  total  induction  through  armature; 

la  the  mean  length  of  lines  of  magnetic  force  in  magnets ; 

Aa  the  area  of  section  of  magnets; 

*  Phil.  Trans.,  188(5;  page  88  of  this  volume. 


DYNAMO-ELECTRIC   MACHINERY.  137 

v  the  ratio  of  induction  in  magnets  to  induction  in  ar- 
mature; 

/  the  function  which  the  magnetizing  force  is  of  the  in- 
duction in  the  case  of  the  machine  actually  taken 
from  Dr.  J.  Hopkinson  on  the  "  Magnetization  of 
Iron,"  Phil.  Trans.,  1885,  Figs.  4  and  5,  Plate  47. 

In  estimating  A^  we  take  the  mean  of  the  diameter  of 
the  core  and  of  the  bore  of  the  magnets  19.8  cms.,  and  the 
angle  subtended  by  the  pole  face  112°,  and  we  add  a  fringe 
all  round  the  area  of  the  pole  face  equal  in  width  to  the 
distance  of  the  core  from  the  pole  face.  This  is  a  wider 
fringe  than  was  used  in  the  earlier  experiments,*  because 
the  form  of  the  magnets  differs  slightly.  The  area  so 
estimated  is  906  sq.  cms. 

Z3  is  taken  to  be  108.8  cms. 

A3  is  435.5  sq.  cms. 

v  was  determined  by  the  ballistic  galvanometer  to  be 
1.47.  It  is  to  be  expected  that,  as  the  core  is  actually 
greater  in  area  than  the  magnets,  v  will  be  more  nearly 
constant  than  in  the  earlier  experiments.  It  was  found  to 
be  constant  within  the  limits  of  errors  of  observation. 

Referring  to  Fig.  45,  the  curve  C  is  the  curve  x  —  I3f  f  -jH, 
and  the  straight  line  B  is  the  curve  x  =  2  /,  -j-,  while  the 

^2 

full  line  D  is  the  characteristic  curve  of  the  machine, 


as  given  by  calculation. 

*  Phil.  Trans.,  1886;  page  95  of  this  volume, 


138    DYNAMO  MACHINERY   AND  ALLIED   SUBJECTS. 

The  marks  -j-  indicate  the  results  of  actual  observations 
on  machine  No.  1,  and  the  marks  0  the  results  on  machine 
No.  2,  the  total  induction  /  being  given  by  the  equation  :— 


potential  difference  in  volts  X  108 
208  X  revolutions  per  second 

Experiments  made  upon  the  power  taken  to  drive  the 
machine  under  different  conditions  show  that  it  takes  about 


Line!  rteeral  of  Mae  letisinaf  Force 


*0000 


250  watts  more  power  to  turn  the  armature  at  660  revolu- 
tions when  the  magnets  are  normally  excited  than  when 
they  are  not  excited  at  all.  The  volume  of  the  core  is 
9,465  cub.  cms.,  or  in  each  complete  cycle  the  loss  per  cubic 

950   ^    ^Q* 

centimetre  is  n  x  9465=  24>000  erg8- 

The  loss  by  hysteresis  is  about  13,000  (Phil.  Trans., 
1885,  p.  463)  if  the  reversals  are  made  by  variation  of  in- 
tensity of  the  magnetizing  force  and  the  iron  is  good 
wrought  iron.  This  result  is  similar  to  that  in  the  earlier 


DYNAMO-ELECTRIC   MACHINERY.  139 

Paper,*  where  it  is  shown  that  the  actual  loss  in  the  core, 
when  magnetized,  is  greater  than  can  be  accounted  for 
by  the  known  value  of  hysteresis. 


EFFECTS   OF   THE   CURRENT   IN   THE   ARMATURE. 

Quoting  from  the  Royal  Society  Paper  [page  103  of  this 
volume], "  The  currents  in  the  fixed  coils  around  the  mag- 
nets are  not  the  only  magnetizing  forces  applied  in  a 
dynamo  machine — the  currents  in  the  moving  coils  of  the 
armature  have  also  their  effect  on  the  resultant  field. 
There  are  in  general  two  independent  variables  in  a 
dynamo  machine — the  current  around  the  magnets  and 
the  current  in  the  armature;  and  the  relation  of  E.  M.  F. 
to  currents  is  fully  represented  by  a  surface.  In  well  con- 
structed machines  the  effect  of  the  latter  is  reduced  to  a 
minimum,  but  it  can  be  by  no  means  neglected.  When  a 
section  of  the  armature  coils  is  commutated,  it  must  inevi- 
tably be  momentarily  short  circuited ;  and  if  at  the  time  of 
commutation  the  field  in  which  the  section  is  moving  is 
other  than  feeble,  a  considerable  current  will  arise  in  that 
section,  accompanied  by  waste  of  power  and  destructive 
sparking.  .  .  . 

"  Suppose  the  commutation  occurs  at  an  angle  A  in  advance 
of  the  symmetrical  position  between  the  fields,  and  that  the 
total  current  through  the  armature  be  C,  reckoned  positive 
in  the  direction  of  the  resultant  E.  M.  F.  of  the  machine, 
i.e.,  positive  when  the  machine  is  used  as  a  generator  of 
electricity.  Taking  any  closed  line  through  magnets  and 

*  See  page  121  of  this  volume. 


140  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 


armature,  symmetrically  drawn  asABCDJS  FA  [Fig.  46], 
it  is  obvious  that  the  line  integral  of  magnetic  force  is  di- 
minished by  the  current  in  the  armature  included  between 
angle  A  in  front  and  angle  X  behind  the  plane  of  symme- 
try. If  m  be  the  number  of  convolutions  of  the  armature, 

the  value  of  this  magnetizing  force  \& 


C  -—  = 


opposed  to  the  magnetizing  force  of  the  fixed  coils  on  the 


Fio.  46. 

magnets.  Thus  if  we  know  the  lead  of  the  brushes  and  the 
current  in  the  armature  we  are  at  once  in  a  position  to  cal- 
culate the  effect  on  the  electromotive  force  of  the  machine. 
A  further  effect  of  the  current  in  the  armature  is  a  material 
disturbance  of  the  distribution  of  the  induction  over  the 


DYNAMO-ELECTRIC   MACHINERY.  141 

bored  face  of  the  pole  piece;  the  force  along  BC  [Fig. 
46]  is  by  no  means  equal  to  that  along  D  E.  Draw  the 
closed  curve  B  C  G  H  B,  the  line  integral  along  0  G,  and  HB 
is  negligible.  Hence  the  difference  between  force  H  G  and 

777    1C 

B  G  is  equal  to  4  n  G  -=  —  =  2  K  m  G,  where  K  is  the  angle 

COG." 

To  verify  this  formula  is  one  of  the  principal  objects  of 
this  Paper. 

A  pair  of  brushes  having  relatively  fixed  positions  near 
together,  and  insulated  from  the  frame  and  from  one 
another,  are  carried  upon  a  divided  circle,  and  bear  upon 
the  commutator.  The  difference  of  potential  between  these 
brushes  was  measured  in  various  positions  round  the  com- 
mutator, the  current  in  the  armature,  the  potential  differ- 
ence of  the  main  brushes,  and  the  speed  of  the  machine 
being  also  noted. 

The  results  are  given  in  Figs.  47,  48,  49,  and  50,  in 
which  the  ordinates  are  measured  potential  differences, 
and  the  abscissae  are  angles  turned  through  by  the  ex- 
ploring brushes.  The  potential  differences  in  Fig.  47 
were  measured  by  a  Siemens  voltmeter,  and  eacli  ordi- 
nate  is  therefore  somewhat  smaller  than  the  true  value, 
owing  to  the  time  during  which  the  exploring  brashes 
were  not  actually  in  contact  with  the  commutator  seg- 
ments. But  this  does  not  affect  the  results,  because  the 
area  is  reduced  in  the  same  proportion  as  the  potential 
differences.  In  Figs.  48,  49,  and  50  the  potential  differ- 
ences were  taken  on  one  of  Sir  William  Thomson's  quad- 
rant electrometers,  and  are  correct. 

Take  Fig.  47,  in  which  machine  No.  1  is  a  generator.     A 


142   DYNAMO   MACHINERY  AND   ALLIED   SUBJECTS. 

centimetre  horizontally  represents  10°  of  lead,  and  the 
ordinates  represent  differences  of  potential  between  the 
brushes.  The  area  of  the  curve  is  61.3  sq.  cms.,  and  repre- 
sents 130  volts  and  a  total  field  of  ?  X  ^  X  10' 

=  4.31  X  10*  lines  of  induction.  This  is,  of  course,  not  the 
actual  field,  which  is  3  per  cent,  greater  on  account  of  the 


+~^* 

>T*~ 

+^ 

*~~~^ 

\ 

f 

*~**^ 

\ 

I 

\^ 

**^ 

6O                              1OO                            ISO                             2OO 
FlO.  47. 

resistance  of  the  armature,  but  is  represented  by  an  area  3 
per  cent,  greater.  An  ordinate  of  1  cm.  will  represent  an 

1    ' '  1 

induction  of  -   -  x  10"  =  7.0  X  10*  lines  in  10°.     The  area 

01. o 

of  10°  is  39.5  X  1.73  =  68.3  sq.  cms.*  Hence  an  ordinate 
of  1  cm.  represents  an  induction  of  1,024  lines  per  square 
centimetre.  The  difference  between  ordinates  at  50°  and 
140°  is  2.5;  hence  the  difference  of  induction  is  actually 
2,560.  Theoretically,  we  have  K  =  £  n  m  =  104  C  =  9.4. 
Therefore  2  K  m  C  =  3,072,  and  this  is  the  line  integral  of 
magnetizing  force  round  the  curve. 

Let  A  be  the  induction  at  50°  and  A  +  6  at  140°:  these 


*  In  calculating  this  area,  the  allowance  for  fringe  at  ends  of  armature  is 
taken  less  than  before,  because  the  form  of  opposing  faces  differs. 


DYNAMO-ELECTRIC    MACHINERY. 


143 


also  are  the  magnetizing  forces.   Hence  (^4  -f  d)  1.4  —  A  1.4 
=  2  x:  w  (7  ;  6  =  2,200,  as  against  2,560  actually  observed. 
Take  Fig.  48,  in  which  No.  2  machine  is  a  motor.     The 


total  field  =         x  .     x  i08  =  5.15  x  10'  lines  of  indue- 
104       /wO 

tion.     Since  the  area  of  the  diagram  is  53.5  sq.  cms.,  an 

5  15 

ordinate  of  1  cm.  =  -^—  X  10"  =  96  X  104  lines  of  induc- 
oo.o 


*-H 

'"'"*—  s 

c 

/ 

t*"*^ 

N 

X 

s 

\ 

0                                 310                               260                             210 
FlO.  48. 

tion  in  10°.     Hence  an  ordinate  of  1  cm.  represents  an 

9.6  X  10* 
induction  of    '    Q  Q —  =  1,400  lines  per  square  centimetre. 

Do.o 

The  difference  between  ordinates  at  320°  and  at  230°  is  2.0; 
hence  the  difference  of  induction  is  actually  2,800.     Theo- 

2*mC      3|  X  104  X  11.4 
retically,  we  have  -  — =  -  -  =  2,666,  as 

I  1.4 

against  2,800  actually  observed. 

In  Fig.  49  No.  1  machine  is  a  generator.     The  total  field 

=  jjjj  X  jig  X  10"  =  3.97  X  10"  lines.     The  area  of  the 
diagram  is  90.9  sq.  cms.,  and  therefore  an  ordinate  of  1  cm. 

O    Qiy 

=  °-  -  x  10"  =  4.37  X  10*  lines  in  10°.     Hence  an  ordi- 


144  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

4  37  X  104 
nate  of  1  cm.  represents  an  induction  of  -  -  =  639 

Oo.O 

lines  per  square  centimetre.     The  difference  between  ordi- 


200 


nates  at  50°  and  at  140°  is  4.5;  hence  the  difference  of 
induction  is  actually  2,877.     Theoretically,  we  have  - 

_  3}  X  104  X  12.9 

1.4 

In  Fig.  50  No.  2  machine  is  a  motor.     The  total  field 
63.5 


x 


"  "  4'9G  x  10"  lines*    The  area  of  the 


diagram  is  112.2  sq.  cms.,  and  therefore  an  ordinate  of  1  cm. 
4.96 


112.2 


X  10'  =  4.42  X  104  lines  in  10°.     Hence  an  ordi- 


4  42 
nate  of  1  cm.  represents  an  induction  of  -    ~  X  104  =  647 

Oo.o 

lines  per  square  centimetre.     The  difference  between  ordi- 
nates  at  323°  and  at   233°   is  4.2;   hence  the  difference 


DYNAMO-ELECTRIC    MACHINERY. 


145 


of  induction  is  actually  2,718.      Theoretically,  we  have 

2*mC      3!  X  104  X  12.3 

— j —  =  -       -j-j =  2,870,  as  against  2,718  act- 
ually observed. 


7 


310 


26O 


810 


FIG.  50. 


At  page  108  of  the  preceding  Paper  on  Dynamo-Electric 
Machinery  it  is  shown  that 


C\ 
J, 


where  /=  F(4  7tnc)is  the  characteristic  curve  when  C=  0, 
and  X  is  the  lead  of  the  brushes. 

The  following  is  an  endeavor  to  verify  this  formula. 
The  potentials  both  upon  the  magnets  and  upon  the 
brushes  were  taken  by  a  Siemens  voltmeter,  and  are  rough. 
The  speeds  were  taken  by  a  Buss  tachometer,  and  there  is 
some  uncertainty'  about  the  precise  lead  of  the  brushes, 
owing  to  the  difficulty  in  determining  the  precise  position 


146  DYNAMO  MACHINERY  AND   ALLIED    SUBJECTS. 


of  the  symmetrical  position -between  the  fields,  and  also  to 
the  width  of  the  contacts  on  the  commutator. 

It  was  necessary,  in  order  to  obtain  a  marked  effect  of 
the  armature  reaction,  that  the  magnet  field  should  bo 
comparatively  small,  that  the  current  in  the  armature 
should  be  large,  and  the  leads  of  the  brushes  should  be 
large. 

The  two  machines  had  their  axles  coupled  so  that  No.  1 
could  be  run  as  a  generator,  and  No.  2  as  a  motor.  The 
magnets  were  in  each  case  coupled  parallel,  and  excited  by 
a  battery  each  through  an  adjustable  resistance.  The  two 
armatures  were  coupled  in  series  with  another  battery,  and 
the  following  observations  were  made : — 


Potential  on 
Magnets  in  volts. 

Potential  on 
Brushes. 

Speed  per 
Minute. 

Current  in 
Amperes. 

Lead  of 
Brushes. 

No.  1 
No.  2 

24-24 
29—29 

86-67 
86-84 

880 
880 

10-2-108 
102-103 

26° 
29° 

Prom  which  we  infer: — 


Current  in 
Magnets. 

4wnc. 

Corrected  Poten- 
tial for  Resistance 
of  Armature. 

Total 
Induction. 
/. 

No.  1 
No.  2 

1.78 
2.15 

8,900 
10,750 

70.8 
80.7 

2.80xlO« 
2.65x10* 

As  there  was  uncertainty  as  to  the  precise  accuracy  of 
the  measurements  of  potential,  it  appeared  best  to  remeas- 
ure  the  potentials  with  no  current  through  the  armature 
with  the  Siemens  voltmeter  placed  as  in  the  last  experi- 
ment. Each  machine  was  therefore  run  on  open  circuit 
with  its  magnets  excited,  and  its  potential  was  measured. 


DYNAMO-ELECTRIC   MACHINERY. 


14? 


Potential  on  Mag- 
nets in  volts. 

Potential  on 
Brushes. 

Speed  per 
Minute. 

Potential  at 
880  Revs. 

No.  I 
No.  2 

25-25 
28—28 

90-90 
79-80 

880 
715-710 

90.0 
98.2 

From  which,  since  the  formula  is  reduced  to 


the  characteristic  being  practically  straight,  we  infer  :  — 


Potential  on 
Magnets. 

Potential  on 
Brushes. 

Induction, 
I=F(4irnc). 

No.  1 
No.  2 

24 
29 

86.4 
101.7 

2.82xlO« 
3.30xlO« 

We  have  further : — 
.\  =  0.45  for  No.  1; 

-  =  2,920; 


A  =  0.5  for  No.  2; 

-4m  C =  443,800. 


4Am(7\ 

4Xm(7 

"~~*4x    r>A'2 

4XmC 

/              4\mC\ 

\                   v     ' 

v 

v             2lt 

V 

\                            v       ' 

^«-^- 

1 

2 

1,314 
1,460 

199,700 
221,900 

7,586 
9,290 

2.41xlO« 
2.90xlO« 

2.21xlO« 
2.68xlO« 

It  has  already  appeared  that  experiment  gives  for  /  in 
No.  1  2.3  X  106,  and  in  No.  2  2.65  X  10*.  The  difference 
is  probably  due  to  error  in  estimating  the  lead  of  the 
brushes,  which  is  difficult,  owing  to  uncertainty  in  the 
position  of  the  neutral  line  on  open  circuit. 


148  DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 


THEORY  OF  ALTERNATING  CURRENTS,  PAR- 
TICULARLY IN  REFERENCE  TO  TWO 
ALTERNATE  CURRENT  MACHINES  CON- 
NECTED TO  THE  SAME  CIRCUIT. 

IN  my  lecture  on  Electric  Lighting,  delivered  before  the 
Institution  of  Civil  Engineers  last  year,*  I  considered  the 
question  of  two  alternate  current  dynamo  machines  con- 
nected to  the  same  circuit,  but  having  no  rigid  mechanical 
connection  between  them ;  and  I  showed  that,  if  two  such 
machines  be  coupled  in  series,  they  will  tend  to  nullify 
each  other's  effect ;  if  parallel,  to  add  their  effects. f  The 
subject  is  one  which  already  has  practical  importance  and 
application,  and  may  have  much  more  in  the  future;  it  is 
also  one  suited  for  discussion,  and  upon  which  discussion 
is  desirable.  I  therefore  venture  to  bring  before  the 
Society  what  I  said  in  my  lecture — some  other  ways  of  look- 
ing at  the  same  subject,  and  an  experimental  verification, 

*  This  Paper  is  reprinted  on  pag*>s  40  to  78  of  this  volume. 

t  November  22,  1884.— My  attention  has  only  to-day  been  called  to  a  paper  by 
Mr.  Wilde,  published  by  the  Literary  nnd  Philosophical  Society  of  Manchester, 
December  15,  1868,  also  Philosophical  Magazine,  January,  1869.  Mr.  Wilde 
fully  describes  observations  of  the  synchronizing  control  between  two  or  more 
alternate  current  machines  connected  together.  I  am  sorry  I  did  not  know  of 
his  observations  when  I  lectured  before  the  Institution  of  Civil  Engineers,  that 
I  might  have  given  him  the  honor  which  was  his  due.  If  his  paper  had  been 
known  to  those  who  have  lately  been  working  to  produce  large  alternate  cur- 
rent machines,  it  would  have  saved  them  both  labor  and  money. 


THEORY  OF  ALTEENATING  CURRENTS.     149 

together  with  solutions  of  other  problems  requiring  similar 
treatment. 

The  general  explanation,  amounting  to  proof  so  far  as 
machines  in  series  are  concerned,  is  given  in  the  following 
extract  from  my  lecture : — 

"  There  remains  one  point  of  great  practical  interest  in 
connection  with  alternate  current  machines:  How  will 
they  behave  when  two  or  more  are  coupled  together  to  aid 
each  other  in  doing  the  same  work  ?  With  galvanic  bat- 
teries we  know  very  well  how  to  couple  them,  either  in 
parallel  circuit  or  in  series,  so  that  they  shall  aid,  and  not 
oppose,  the  effects  of  each  other;  bnt  with  alternate  cur- 
rent machines,  independently  driven,  it  is  not  quite  ob- 
vious what  the  result  will  be,  for  the  polarity  of  each 
machine  is  constantly  changing.  Will  two  machines 
coupled  together  run  independently  of  each  other,  or  will 
one  control  the  movement  of  the  other  in  such  wise  that 
they  settle  down  to  conspire  to  produce  the  same  effect,  or 
will  it  be  into  mutual  opposition  ?  It  is  obvious  that  a 
great  deal  turns  upon  the  answer  to  this  question,  for  in 
the  general  distribution  of  electric  light  it  will  be  desirable 
to  be  able  to  supply  the  system  of  conductors  from  which 
the  consumers  draw  by  separate  machines,  which  can  be 
thrown  in  and  out  at  pleasure.  Now  I  know  it  is  a  com- 
mon impression  that  alternate  current  machines  cannot  be 
worked  together,  and  that  it  is  almost  a  necessity  to  have 
one  enormous  machine  to  supply  all  the  consumers  draw- 
ing from  one  system  of  conductors.  Let  us  see  how  the 
matter  stands.  Consider  two  machines  independently 
driven,  so  as  to  have  approximately  the  same  periodic  time 
and  the  same  electromotive  force,  If  these  two  machines 


150   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

are  to  be  worked  together,  they  may  be  connected  in  one 
of  two  ways :  they  may  be  in  parallel  circuit  with  regard 
to  the  external  conductor,  as  shown  by  the  full  line  in 
Fig.  51,  that  is,  their  currents  may  be  added  algebraically 
and  sent  to  the  external  circuit,  or  they  may  be  coupled  in 
series,  as  shown  by  the  dotted  line,  that  is,  the  whole  cur- 
rent may  pass  successively  through  the  two  machines,  and 
the  electromotive  force  of  the  two  machines  may  be  added, 


instead  of  their  currents.  The  latter  case  is  simpler.  Let 
us  consider  it  first.  I  am  going  to  show  that  if  you  couple 
two  such  alternate  current  machines  in  series  they  will  so 
control  each  other's  phase  as  to  nullify  each  other,  and 
that  you  will  get  no  effect  from  them;  and,  as  a  corollary 
from  that,  I  am  going  to  show  that  if  you  couple  them  in 
parallel  circuit  they  will  work  perfectly  well  together,  and 
the  currents  they  produce  will  be  added;  in  fact,  that  you 


THEORY  OF  ALTERNATING  CURRENTS. 


151 


cannot  drive  alternate  current  machines  tandem,  but  that 
you  may  drive  them  as  a  pair,  or,  indeed,  any  number 
abreast.  In  diagram,  Fig.  52,  the  horizontal  line  of  abscissae 
represents  the  time  advancing  from  left  to  right;  the  full 
curves  represent  the  electromotive  forces  of  the  two 
machines  not  supposed  to  be  in  the  same  phase.  We  want 
to  see  whether  they  will  tend  to  get  into  the  same  phase  or 
to  get  into  opposite  phases.  Now,  if  the  machines  are 
coupled  in  series,  the  resultant  electromotive  force  on  the 
circuit  will  be  the  sum  of  the  electromotive  forces  of  the 


nur 


two  machines.  This  resultant  electromotive  force  is  rep- 
resented by  the  broken  curve  ///;  by  what  we  have  already 
seen  in  Formula  IV.  [p.  52,  this  volume],  the  phase  of 
the  current  must  lag  behind  the  phase  of  the  electro- 
motive force,  as  is  shown  in  the  diagram  by  curve  IV,  thus 

. . .     Now  the  work  done  in  any  machine   is 

represented  by  the  sum  of  the  products  of  the  currents 
and  of  the  electromotive  forces,  and  it  is  clear  that,  as  the 
phase  of  the  current  is  more  near  to  the  phase  of  the  lag- 
ging machine  //  than  to  that  of  the  leading  machine  /,  the 
lagging  machine  must  do  more  work  in  producing  elec- 


152   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

tricity  than  the  leading  machine;  consequently  its  velocity 
will  be  retarded,  and  its  retardation  will  go  on  until  the 
two  machines  settle  down  into  exactly  opposite  phases, 
when  no  current  will  pass.  The  moral,  therefore,  is,  do  not 
attempt  to  couple  two  independently  driven  alternate  cur- 
rent machines  in  series.  Now  for  the  corollary:  A,  B, 
Fig.  51,  represent  the  two  terminals  of  an  alternate  cur- 
rent machine;  a,  b,  the  two  terminals  of  another  machine 
independently  driven.  A  and  a  are  connected  together, 
and  B  and  b.  So  regarded,  the  two  machines  are  in  series, 
and  we  have  just  proved  that  they  will  exactly  oppose  each 
other's  effects,  that  is,  when  A  is  positive,  a  will  be 
positive  also;  when  A  is  negative,  a  is  also  negative. 
Now,  connecting  A  and  a  through  the  comparatively  high 
resistance  of  the  external  circuit  with  B  and  b,  the  cur- 
rent passing  through  that  circuit  will  not  much  disturb,  if 
at  all,  the  relations  of  the  two  machines.  Hence,  when  A 
is  positive,  a  will  be  positive,  and  when  A  is  negative,  a 
will  be  negative  also;  precisely  the  condition  required  that 
the  two  machines  may  work  together  to  send  a  current 
into  the  external  circuit.  You  may,  therefore,  with  con- 
fidence, attempt  to  run  alternate  current  machines  in 
parallel  circuit  for  the  purpose  of  producing  any  external 
effect.  I  might  easily  show  that  the  same  applies  to  a 
larger  number;  hence  there  is  no  more  difficulty  in  feed- 
ing a  system  of  conductors  from  a  number  of  alternate 
current  machines  than  there  is  in  feeding  it  from  a  num- 
ber of  continuous  current  machines.  A  little  care  only  is 
required  that  the  machine  shall  be  thrown  in  when  it  has 
attained  something  like  its  proper  velocity.  A  further 
corollary  is  that  alternate  currents  with  alternate  current 


THEORY  OF  ALTERNATING  CURRENTS. 


153 


machines   as   motors   may  theoretically  be  used   for  the 
transmission  of  power."  * 

Although  the  proof  of  this  corollary  regarding  motors  is 
similar  to  what  we  have  just  been  going  through,  it  may 
be  instructive  to  give  it.  In  the  accompanying  diagrams, 
Figs.  53  and  54,  the  full  lines  /  and  21  represent  the 


Fio.68. 

electromotive  forces  of  the  two  machines  (generator  and 
receiver) ;  the  dotted  line,  curve  777  (.  .  .  .),  the  resultant 
electromotive  force;  and  the  curve  IV,  the  resulting  cur- 
rent, each  in  terms  of  the  time,  as  abscissae.  The  only  dif- 
ference between  the  two  diagrams  is,  that  in  Fig.  53  the 
two  machines  have  equal  electromotive  forces,  while  in 
Fig.  54  the  receiving  machine  has  double  the  electromotive 
force  of  the  generator.  In  both  figures  the  receiving 
machine  lags  behind  the  phase  of  direct  opposition  to  the 
generator  by  one  quarter  of  a  period,  or  something  less. 
Now  observe,  the  resultant  electromotive  force  must  be  in 


*  "  Of  course  in  applying  these  conclusions  it  is  necessary  to  remember 
that  the  machines  only  tend  to  control  each  other,  and  that  the  control  of  the 
motive  power  may  be  predominant  and  compel  the  two  or  more  machines  to 
run  at  different  speeds." 


154   DYNAMO   MACHINERY   AND    ALLIED   SUBJECTS. 

phase  behind  the  receiver,  but  in  advance  of  the  generator. 
Also  observe,  the  current  must  be  in  phase  behind  the  re- 
sultant electromotive  force,  and  may  be  one  quarter  of  a 
period  behind,  provided  only  the  self  induction  be  large 
enough  compared  with  the  resistance.  The  current  will 
then  be  less  than  a  quarter  period  behind  the  generator. 
This  machine  will  do  work  upon  the  current,  but  the  cur- 


<J 


FIG.  54. 


rent  will  be  more  than  a  quarter  period  behind  the  receiv- 
ing machine;  therefore  in  the  receiver  the  current  does 
work  upon  the  machine. 

The  subject  is  illustrated  by  the  following  problems.  Of 
course  any  of  them  may  be  treated  more  generally  by  con- 
sidering the  machines  as  unequal,  or  by  introducing  other 
periodic  terms,  but  I  do  not  see  that  this  would  throw  more 
light  on  the  subject: — 

I.  Two  alternate  current  machines,  equal  in  all  respects, 
are  connected  in  series  and  independently  driven  at  the 
same  speed,  to  determine  the  current,  etc.,  in  each. 


THEORY   OF  ALTERNATING   CURRENTS.  155 

Let  y  be  the  coefficient  of  self  induction  of  each,  r  the 

2  n 
resistance,  x  the  current  at  time  t,  and  E  sin  —=-  (t  -\-  T) 

2# 

and  E  sin  -^  (t  —  r)    the  electromotive   forces.      Then 

regarding  the  coefficient  of  self  induction  as  constant, 
which  it  is  not  exactly,  and  neglecting  the  effect  of  currents 
other  than  those  in  the  copper  wire,  the  equation  of  motion  is 


=  E  \  sm  -^  (t  +  i 

27r£         2  TT  r 
or  ^  a; '+  r  x  =  12  sm  — ~-  cos     ^    ; 

whence 

I  p  27TT  V 

~^~      (  2  TT/        27TX         2?r# 

a=  /9  ^  -,\»    •!  »•  sm  —fff-  +  —^-  cos  -^r- 


Work  done  by  the  leading  machine  per  second 


~T~        (  ZTTT      Zxy   . 


From  this  it  at  once  follows  that  the  leading  machine  does 

T 

least  work,  and  will  tend  to  increase  its  lead  until  r  =  -  , 


156  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

when  the  two  machines  will  neutralize  each  other,  as  al- 
ready proved  geometrically.  The  leading  machine  may  ac- 
tually become  a  motor  and  do  mechanical  ivorJc,  although 
its  electromotive  force  is  precisely  equal  to  that  of  the  fol- 
loiving  machine. 

Considering  the  important  case  when  r  is  negligible,  we 
have 


ZTTT 
E  cos  —-.  cos 


X  = 


27TX 

T~ 


T 

rate  of  working   =  -  — . 

4.'2|Z 

T 
This  is  the  maximum  when  r  =  -,  and  then  it  is  equal  to 

o 

the  maximum  work  which  can  be  obtained  from  either 
machine  when  connected  to  a  resistance  only,  which  occurs 

when  that  resistance  is  ~  ;  the  current,  also,  is  the  same 
as  when  the  maximum  work  is  being  done  on  resistance, 
and  is  —  of  the  current  the  machine  will  give  if  short 

circuited.  The  difference  of  potential  between  the  two 
leads  connecting  the  machines,  whether  r  =  0  or  not,  is 

E  cos  ^-Tff-  sin  —jfr  •     If  there  be  no  work  done  on  the  re- 

T 

ceiving  machine  and  r  =  0,  T  —  -,  and  the  amplitude  of 


THEORY    OF    ALTERNATING   CURRENTS.  157 

the  difference  of   potential   between   the  leads  is  E\    if, 
on  the  other  hand,  the  maximum  work  is  being  transmitted, 

the  potential  measured  will  be  —  of  that   observed   when 

either  machine  is  run  on  open  circuit. 

II.  Two  machines  are  coupled  parallel  and  connected  to 
an  external  circuit  resistance  R. 

Let  xl  ,  xt  be  currents  in  the  two  machines.  The  ex- 
ternal  current  will  be  xl  -f  #a  ,  and  consequently  the  differ- 
ence of  potential  at  the  junction,  R  (xl  -J-  #3). 

Let  the  electromotive  forces  of  the  two  machines  regarded 

in  this  case  as  connected  parallel  be  E  sin  -    -i=  --  -,  and 

let  the  self  induction  and  resistance  of  each  be  $-y  and  2  r. 
The  equations  of  motion  then  are  : 


=E  sin  -  R  (x, 

whence 


and 


\  T7  ^   7ft       .         2    TIT 

-  X    =  E  cos  --  sm  --, 


168  DYNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 
Solving  these, 

27TT 


•     2  arr 


Electrical  work  done  by  the  leading  machiue 


7f  y     ,          7T  T  7TT  ) 

--  7^-  Bin  -^-  COS  —;£-   j, 


%7T  V     .      % 


THEORY  OF  ALTERNATING  CURRENTS.         159 

This  expression  shows  that  the  leading  machine  does  most 
work  in  all  cases.     Suppose  r  is  small  compared  with  R 

~ 


and  —,  also  that  R  =  —jr~>  we  have  the  work  done 
per  second 


T 

Make  r  =  — --,  and  we  see  that  the  following  machine 
o 

will  then  do  no  work;  when  T  exceeds  this,  the  following 
machine  becomes  a  motor  and  absorbs  electrical  work. 

III.  Suppose  the  terminals  of  an  alternate  current  ma- 
chine are  connected  to  a  pair  of  conductors,  the  difference 
of  potential  between  which  is  completely  controlled  by  con- 
nection with  other  alternate  current  machines. 

Let  y  and  R  be  the  coefficient  of  self  induction  and  the 
resistance  of  the  machine  and  its  own  conductors  up  to  the 
point  at  which  the  potential  is  completely  controlled.  Let 
the  difference  of  potential  of  the  main  conductors  be 

A  sin  —p-  ,  and  let  the  electromotive  force  of  the  machine 

,      D    .     27f(t-r) 
be  B  sin  -  -^ '- . 

Equation  of  motion  is 

2  n  (t  —  r)  2  7ft 

yx'  +  Rx  =  B  sm ^ *  —  A  sm  — 7fr, 


160  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS, 
whence 

_**(t-r)      *ny 


cos 


;r(*-T))  j  27T*      2*x        2*n~| 

-^r-  ^  j-  -  ^4  |  R  sin  -^-      —  ^-  cos  -^-  j-  J, 

Electrical  work  done  by  the  machine  in  unit  of  time 


- 

=  x  B  sin  -       ,.,      -  = 

T 


If  T  be  positive,  that  is,  if  machine  be  lagging  in  its  phase, 
work  done  is  less  than  if  it  be  negative;  hence  T  will  tend 
to  zero,  or  the  machine  will  tend  to  adjust  itself  to  add  its 
currents  to  that  of  the  system  of  conductors.  The  machine 
may  act  as  a  motor  even  though  its  electromotive  force  be 
greater  than  that  of  the  system,  for  let 


R  %7T  0 

-  =  tan 


7' 


THEORY  OF  ALTERNATING  CURRENTS.         161 
work  (electric)  done  by  machine 


AB 


T 


T 

this  has  a  minimum  value  when  <f>  +  f  =  -j- ,  and  then 

the  mechanical  work  done  by  machine  or  electrical  work 
received  by  the  machine 

B  ( RB  } 


and  this  is  positive,  provided 
•••      '  i>  * 


There  are  two  or  three  other  problems  of  sufficient  in- 
terest to  make  it  worth  while  giving  them  here,  although 
not  directly  relating  to  alternate  current  machines  coupled 
together. 

IV.  To  determine  the  law  of  an  alternate  current 
through  an  electric  arc. 

It  has  been  shown  by  Joubert  that  in  an  arc  the  differ- 
ence of  potential  is  of  approximately  constant  numerical 


162  I>YNAMO  MACHINERY  AND  ALLIED  SUBJECTS. 

value,  reversing  its  value  discontinuously  with  the  reversal 
of  the  current,  probably  at  the  instant  of  reversal  of  cur- 
rent. We  shall  assume,  then,  that  there  is  in  the  arc  a 
constant  electromotive  force,  A,  always  opposed  to  the 
current,  except  when  the  current  ceases,  and  that  then  its 
value  is  zero. 

The  equation  of  motion  is 

yx'  +  ltx  =  EBin^^A, 

the  negative  sign  being  taken  when  x  is  +  **>  the  positive 
when  x  is  negative.  Solving  generally, 

A  E  /      27ty        2?rt 

%  —    ~T~  ~/S     I      /rt  _    ..  \  «  I  7fi~   COS       rn 


This  equation  will  continuously  hold  good  for  a  half  period 
from  x  =  0  to  x  =  0  again,  but  at  each  half  period  the 
arbitrary  constant  C  is  changed  with  the  sudden  change  of 
sign  of  A.  It 'is  determined  by  the  consideration  that  if, 
for  a  certain  value  t0  of  t,  x  should  vanish,  it  shall  vanish 

T 

again  when  t  =  t0  -f-  --- .    This  applies  to  the  case  when  E 

A 

is  sufficiently  large,  as  is  practically  the  case;  but  if  the 
current  should  cease  for  a  finite  time  this  condition  will  be 
varied,  and  instead  of  it  we  have  the  condition  x  =  0  when 

9  "jf  / 

E  sin  -=-  =  A.  This  latter  case  I  do  not  propose  to 
consider  further. 


THEORY   OF   ALTERNATING   CURRENTS.          163 

Let 

2  n  v  %7tt. 


T 

Putting  *  =  t0  and  t  =  t,  +  -^ ,  we  have 


_R  RT 

Ce   *'°  .e     2Y* 


equations  to  determine  t0  and  C. 
Eliminating  C, 

RE'  .27 

- .  sin  - 


164  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

Having  obtained  t0,  C  is  given  by  equation 
o   A  R     I  R 


This  gives  the  complete  solution  of  the  problem. 

A  case  of  special  importance  is  that  in  which  R  is  small ; 
let  us  therefore  consider  the  case  R  =  0;  the  solution 
then  is 

T  „       2  nt 
yx=  —  - —  .tf  cos  — -~ A  t  +  C. 

In  the  same  way  as  before, 

2  7t  t          A   71 


c=. 

The  limiting  case  to  which  the  solution  applies  is  given  by 
x'  =  0  when  *  =  *.  +  £ 

2  7f  t0 

Ci% 
^  ~^~  T)' 

or  A  =  E  X  0.538. 

Roughly,  we  may  say  that,  in  order  that  the  current  may 
not  cease  for  a  finite  time,  E  must  be  at  least  double  of  A; 


THEORY   OF  ALTERNATING   CURRENTS.  165 

A  will  of  course  depend  upon  the  length  of  the  arc.  The 
work  done  in  the  arc  will  be  proportional  to  the  arith- 
metical mean  value  of  the  current  taken  without  regard  to 
sign.  This  is  of  course  quite  a  different  thing  from  the 
mean  current  as  measured  by  an  electro-dynamometer. 
Let  us  examine  what  error  is  caused  by  estimating  the 
work  done  in  the  arc  as  equal  to  the  current  measured  by 
the  dynamometer  multiplied  by  the  mean  difference  of 
potential. 

The  actual  work  done  per  second 
,  T 


The  mean  square  of  the  current  as  measured  by  the  elec- 
tro-dynamometer is 


2    /^o 

f  I 

*/*• 


and  the  work  done  by  this  current  is  apparently  the  square 
root  of  the  above  expression  multiplied  by  A.  It  is  easy 
to. see  that  this  is  greater  in  all  cases  than  the  work  done, 
but  it  is  worth  while  to  examine  the  extent  of  the  error. 
If  we  treated,  the  arc  as  an  ordinary  resistance,  we  should 
assume  work  per  second 


166   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

2 

Taking  a  fairly  practical  case,  assume  A  =  —  E;  we  have 

actual  work  per  second 

A*T    1 


work  done  estimated  by  electro-dynamometer 


25 


=  ^ay  1        7235 

X    SOY/  12"' 

or  nearly  £  part  too  much.  This  will  suffice  to  show  that 
the  matter  is  not  a  mere  theoretical  refinement.  Another 
erroneous  method  of  estimating  the  power  developed  in  an 
arc  is  to  replace  by  a  resistance  and  adjust  this  resistance 
till  the  current  as  measured  by  an  electro-dynamometer 
is  the  same  as  with  the  arc,  and  assume  that  the  work  done 
in  the  resistance  is  the  same  as  the  work  done  in  the  arc. 
Returning  to  the  expression 


ny  2 

we  may  inquire,  given  T  A  and  the  dimensions  of  the  ma- 
chine, how  ought  it  to  be  wound  or  its  coils  connected  that 


THEORY  OF  ALTERNATING  CURRENTS.    167 

most  work  may  be  done  in  the  arc.  If  the  number  of  con- 
volutions be  varied,  E  will  vary  as  the  convolutions,  y  as 
their  square;  therefore  y  oc  E* ;  we  are  therefore  to  deter- 
mine E  so  that  •=  A  IE* is  a  maximum  which 

occurs  when  E  =  n  A.  When  the  resistance  of  the  circuit 
is  taken  into  account,  this  result  will  be  modified.  It 
suffices  to  prove  that  it  is  desirable  that  the  potential  of 
the  machine  should  be  materially  in  excess  of  that  required 
to  maintain  the  arc. 

V.*  In  all  that  precedes  it  is  assumed,  not  only  that  y  is 
constant,  but  that  the  copper  conductor  of  the  armature  is 
the  only  conductor  moving  in  the  field.  If  there  be  iron 
cores  in  the  armature,  we  shall  approximate  to  the  effect 
by  regarding  such  cores  as  a  second  conducting  circuit. 
Slightly  changing  the  notation,  let  L  be  coefficient  of  self 
induction  of  the  copper  circuit,  N  coefficient  of  self  induc- 
tion of  the  iron  circuit  and  Rl  its  resistance,  I1  the  mag- 
netic induction  of  the  field  magnets  upon  the  iron  circuit, 
and  M  the  coefficient  of  mutual  induction  of  the  two  cir- 
cuits, y  the  current  in  the  iron.  The  equations  of  motion 
are  obtained  from  the  expression  for  the  energy,  viz., 

%{Lx*  -j-  %Mxy  +  Ny*  —  2  Ix  —  2 1' y}, 
and  are 

r  ~/    i     i,r    /    i     n  d  I       2  7t  A          27ft 

L  x'  -f  M  y'  -f-  R  x  =  -J-T  =  — =,—  cos  — =- , 

d  P      27fS        27ft 
MX'  4-  Nyr  +  R'u  =  -=-r  =     m    cos  ^-, 


*  Fide  also  "Encyclopaedia  Britannica,"  article  "  Lighting." 


168  DYNAMO   MACHINERY  AND   ALLIED  SUBJECTS. 

for  in  general  the  iron  cores  and  the  copper  conductor  are 
symmetrically  arranged.    Assume 


x  =  a  sm 


. 
=  b  cos 


27ft 


,     .       27ft 

=  a'  sm  --      b'  cos 


and  substitute  in  the  equations  of  motion;  we  have  the 
following  four  equations  to  determine  the  constants  «,  b, 
a',  £':— 


A 


or 


and 


=  0, 


=  0. 


These  equations  contain  the  solution  of  the  problem,  but 
are  too  cumbersome  to  be  worth  while  solving  generally; 


THEORY   OF  ALTERNATING   CURRENTS.          169 

we  will,  however,  prove  the  statements  made  in  the  lecture 
before  the  Civil  Engineers. 

1.  Compare  short  circuit  and  open  circuit,  that  is,  R  =  0 
very  nearly,  and  R  =  oc*.     In  the  former  case 'we  find  that 

work  done  in  the  iron  is  diminished,  and  if  B  =  — =r-  we 

lj 

have  the  paradoxical  result  that  there  are  no  currents  in- 
duced in  the  iron  of  the  cores  and  no  work  is  required  to 
drive  the  machine.  This,  of  course,  can  never  actually 
occur,  because  R  can  never  absolutely  vanish.  It  suffices 
to  show,  however,  that  the  current  in  the  copper  circuit 
may  diminish  the  whole  power  required  to  drive  the  ma- 
chine to  an  amount  less  than  the  power  required  to  drive 
the  machine  on  open  circuit. 

2.  The  other  statement  related  to  the  effect  of  the  cur- 
rents in  the  iron  upon  the  currents  produced  in  the  copper 
circuit.     Assume  that  the  effect  is  a  small  one,  for  a  first 
approximation.     Neglect  it,  that  is,  treat  the  currents  in 
the  iron  and  the  currents  in  the  copper  as  independent  of 
each  other,  and  then  see  how  each  would  disturb  the  other. 

The  first  approximation  then  is 


AL  ,_         EN 

®  /T»2  r>2)      **    '  rrty  pf2> 

£'+^4       ^j+^-i- 


.TR  TR' 

~ 


7,- 

" 


170  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

If  we  substitute  these  in  the  general  equations  as  correc- 
tions, we  have 


4;r« 


which  shows  that  the  disturbing  effect  of  each  circuit  upon 
the  other  is  to  diminish  the  apparent  electromotive  force, 
but  to  accelerate  its  phase. 

VI.  A  very  similar  problem  is  that  of  secondary  genera- 
tors or  induction  coils,  whether  used  for  the  conversion  of 
high  potentials  to  low,  or  the  reverse.  To  treat  it  gener- 
ally, taking  the  magnetization  of  the  iron  cores,  which  are 
always  used,  as  a  non-linear  function  of  the  currents  in  the 
coils,  would  be  a  matter  of  much  difficulty;  we  therefore 
assume,  as  is  usual,  that  the  coefficients  of  induction  are 
constants,  noting  in  passing  that  this  is  not  strictly  the 
fact,  though  it  is  very  nearly  the  fact,  when  the  cores  are 
not  saturated  and  when  the  lines  of  magnetic  induction 
pass  through  non-magnetic  space. 

Let,  then,  R,  r  be  the  resistances  of  the  primary  and 
secondary  circuits; 

L  coefficient  of  self  induction  of  the  primary; 

^coefficient  of  self  induction  of  the  secondary; 

M  coefficient  of  mutual  induction  of  the  two  circuits; 


THEORY   OF   ALTERNATING   CURRENTS.          171 

x  and  y  the  currents  in  the  two  circuits  at  time  t; 

JTihe  electromotive  force  applied  in  the  primary  circuit 
by  an  alternate  current  dynamo  machine  or  other- 
wise; 
the  equations  of  motion  will  be 

Lx'  +  My'  +  Rx  =  X, } 

MX'  +  Ny'  +  ry  =  0. ) 

Various  assumptions  may  be  made  as  to  X,  but  that  most 
likely  to  be  adopted  in  the  practical  work  of  secondary 
generators  is  that  X  is  kept  so  adjusted  that 


2  n 
x  =  A  cos  n  t  where  n  =  — , 


and  to  inquire  how  X  will  depend  on  the  resistances 
Ny'  +  r  y  =  n  A  M  sin  n  t, 

y  =    a  -»ra    . — *  (  —  n  N  cos  n  t  +  r  sin  n  t), 
9      if  N*  4-  ra  v  ' 


8. 

N 


172   DYNAMO   MACHINERY    AND   ALLIED   SUBJECTS. 

As  in  the  case  of  the  dynamo  machine,  the  work  done  in 
the  secondary  circuit  is  greatest  when  r  =  n  N.  The  ex- 
pression for  X  serves  to  show  that  when  the  secondary  is 
short  circuited  a  loiver  electromotive  force  of  the  generat- 
ing circuit  is  required  than  when  it  is  on  open  circuit.  In 
induction  coils  the  electrostatic  capacity  of  the  coils  them- 
selves has  important  effects.  An  illustration  of  the  effect 
of  electrostatic  induction  is  found  in  the  old-fashioned 
Ruhmkorff  coils.  These  were  not  wound  symmetrically,  but 
in  such  wise  that  one  end  of  the  secondary  coil  was  on  the 
whole  towards  the  inside,  the  other  towards  the  outside  of 
the  bobbin.  In  such  coils  a  spark  to  earth  may  be  obtained 
from  the  outside  end,  but  not  from  the  inside.  The  reason 
is  that  the  outer  convolutions  have  smaller  electrostatic 
capacity  than  the  inner  ones.  The  terminals  may  be  made 
to  give  equal  sparks  by  the  simple  expedient  of  laying  a 
piece  of  tinfoil  around  the  whole  coil  and  connecting  it  to 
earth. 

VII.  Some  time  ago  Dr.  Muirhead  told  me  that  he  had 
observed  that  the  effect  of  an  alternate  current  machine 
could  be  increased  by  connecting  it  to  a  condenser.  This 
is  not  difficult  to  explain :  it  is  a  case  of  resonance  anal- 
ogous to  those  which  are  so  familiar  in  the  theory  of  sound 
and  in  many  other  branches  of  physics. 

Take  the  simplest  case,  though  some  others  are  almost 
as  easy  to  treat.  Imagine  an  alternate  current  machine 
with  its  terminals  connected  to  a  condenser;  it  is  required 
to  find  the  amplitude  of  oscillation  of  potential  between  the 
two  sides  of  the  condenser.  Let  R  y  be  the  resistance  and 

2  71  t 

self  induction  of  the  machine,  E  sin  -~-  its  electromotive 


THEORY    OF   ALTERNATING   CURRENTS.          173 

force,  C  the  capacity  of  the  condenser,  V  the  difference  of 
potential  sought,  and  x  the  current  in  the  machine ;  then 

C  V  =  x, 
and 


_          ,  f*     f+    V  -— 

~~    -*-*     D  fwj  W     J 

whence 


.    „„%  X   .    27ft 
+  R  0-     sin 


.    .  .  T  T    27TEC 


amplitude  of  V  is  therefore 


Now  suppose  E  =  100  volts,  the  machine  would  light  up 
an  incandescent  lamp  of  about  69  volts.   Let  T  =  -%  fa  second, 


174  DYNAMO  MACHINERY   AND   ALLIED  SUBJECTS. 

2  n  y 
C  =  100  microfarads,  and  — =~  =  8  ohms,  and  R  =  -fa 

ohm,  all  figures  which  could  be  practically  realized;  we 
have  amplitude  of  V  =  80  E  roughly,  or  the  apparent 
electromotive  force  would  be  increased  eighty  fold. 

We  now  return  to  the  principal  subject  of  the  present 
communication.  Some  attempts  have  been  made  to  verify 
the  proposition  that  two  alternate  current  machines  can 
be  advantageously  connected  parallel,  but,  I  believe,  till 
recently  without  success.  I  had  no  convenient  opportu- 
nity for  testing  the  point  myself  till  last  summer,  when  I 
had  two  machines  of  De  Meritens,  intended  for  the  light- 
house of  Tino,  in  my  hands.  I  have  made  no  determina- 
tions of  the  constants  of  these  machines,  but  between  three 
and  four  years  ago  I  thoroughly  tested  a  pair  of  similar 
machines  now  in  use  at  a  lighthouse  in  New  South  Wales. 
Each  machine  haa  five  rings  of  sixteen  sections,  and  forty 
permanent  magnets.  The  resistance  of  the  whole  machine 
as  connected  for  lighthouse  work  (a  single  arc)  was  0.0313, 
its  electromotive  force  (E )  when  running  830  revolutions 

per  minute,  95  volts  and  (— ar-J  =  0.044  ohm.      It  was 

further  remarked  that  the  loss  of  power  was  least  with  a 
maximum  load,  as  is  shown  in  the  following  table: — 

Power  applied  as  measured  in  belt 3.1         4.8         5.6        6.5        5.4 

Electric  power  developed 0.7         8.4         4.3         5.7         3.4 

Mean  current  in  amperes 7.7       38.6       51.7       73.6        151 

This  result   illustrates  well  the  conclusion  arrived  at  in 
Problem  V.  above. 
Last  summer  the  two  machines  for  Tino  were  driven 


THEORY   OF  ALTERNATING  CURRENTS.         175 

from  the  same  countershaft  by  link  bands,  at  a  speed  of 
850  to  900  revolutions  per  minute;  the  pulleys  on  the 
countershaft  were  sensibly  equal  in  diameter,  but  those  on 
the  machines  differed  by  rather  more  than  a  millimetre, 
one  being  300,  the  other  299  mms.  in  diameter  (about); 
thus  the  two  machines  had  not  when  unconnected  exactly 
the  same  speed.  The  pulleys  have  since  been  equalized. 
The  bands  were  of  course  put  on  as  slack  as  practicable, 
but  no  special  appliance  for  adjusting  the  tightness  of  the 
bands  was  used.  The  experiment  succeeded  perfectly  at 
the  very  first  attempt.  The  two  machines,  being  at  rest, 
were  coupled  in  series  with  a  pilot  incandescent  lamp 
across  the  terminals;  the  two  bands  were  then  simulta- 
neously thrown  on :  for  some  seconds  the  machines  almost 
pulled  up  the  engine.  As  the  speed  began  to  increase,  the 
lamp  lit  up  intermittently,  but  in  a  few  seconds  more  the 
machines  dropped  into  step  together,  and  the  pilot  lamp 
lit  up  to  full  brightness  and  became  perfectly  steady  and 
remained  so.  An  arc  lamp  was  then  introduced,  and  a  per- 
fectly steady  current  of  over  200  amperes  drawn  off  with- 
out disturbing  the  harmony.  The  arc  lamp  being  removed, 
a  Siemens  electro-dynamometer  was  introduced  between 
the  machines,  and  it  was  found  that  the  current  passing 
was  only  18  amperes,  whereas,  if  the  machines  had  been  in 
phase  to  send  the  current  in  the  same  direction,  it  would 
have  been  more  than  ten  times  as  great.  On  throwing  off 
the  two  bands  simultaneously,  the  machines  continued  to 
run  by  their  own  momentum,  with  retarded  velocity.  It 
was  observed  that  the  current,  instead  of  diminishing  from 
diminished  electromotive  force,  steadily  increased  to  about 
50  amperes,  owing  to  the  diminished  electrical  control  be- 


176  DYNAMO  MACHINERY   AND   ALLIED   SUBJECTS. 

tween  the  machines,  and  then  dropped  off  to  zero  as  the 
machines  stopped.  Professor  Adams  will,  I  hope,  give  an 
account  of  experiments  he  has  tried  wifh  me,  and  on  other 
occasions,  at  the  South  Foreland.  With  De  Meritens' 
machines,  I  regard  coupling  two  or  more  machines  parallel 
as  practically  the  best  way  of  obtaining  exceptionally  great 
currents  when  required  in  a  lighthouse  for  penetrating  a 
thick  atmosphere. 


AN  UNNOTICED  DANGER.  177 


AN  UNNOTICED  DANGER  IN  CERTAIN  APPA- 
RATUS FOR  DISTRIBUTION  OF  ELECTRICITY. 

MANY  plans  have  been  proposed,  and  several  have  been 
to  a  greater  or  less  extent  practically  used,  for  combining 
the  advantage  of  economy  arising  from  a  high  potential  in 
the  conductors  which  convey  the  electric  current  from  the 
place  where  it  is  generated  with  the  advantages  of  a  low 
potential  at  the  various  points  where  the  electricity  is  used. 
A  low  potential  is  necessary  where  the  electricity  is  used ; 
partly  because  the  lamps,  whether  arc  or  incandescent,  each 
require  a  low  potential,  and  partly  because  a  high  potential 
may  easily  become  dangerous  to  life.  Among  the  plans 
which  have  been  tried  for  locally  transforming  a  supply  of 
high  potential  to  a  lower  and  s,afer,  the  most  promising  is 
by  the  use  of  secondary  generators  or  induction  coils.  It 
has  been  proved  that  this  method  can  be  used  with  great 
economy  of  electric  power  and  with  convenience;  under 
proper  construction  of  the  induction  coils  it  may  also  be 
perfectly  safe.  It  is,  however,  easy  and  very  natural  so  to 
construct  them  that  they  shall  be  good  in  all  other  respects 
but  that  of  safety  to  life — that  they  shall  introduce  an  un- 
expected risk  to  those  using  the  supply. 

In  a  distribution  of  electricity  by  secondary  generators, 
an  alternating  current  is  led  in  succession  through  the 
primary  coils  of  a  series  of  induction  coils,  one  for  each 


178  DYNAMO  MACHINERY  AND  ALLIED 

group  or  system  of  lamps.  The  lamps  connect  the  two 
terminals  of  the  secondary  coil  of  the  induction  coils.  It 
is  easy  to  so  construct  the  induction  coils  that  the  differ- 
ence of  potential  between  the  terminals  of  the  secondary 
coils  may  be  any  suitable  number  of  volts,  such  as  50  or 
100;  while  the  potential  of  the  primary  circuit,  as  meas- 
ured between  the  terminals  of  the  dynamo  machine,  may 
be  very  great,  e.g.,  2,000  or  3,000  volts.  If  the  electromag- 
netic action  between  the  primary  and  secondary  coils,  on 
which  the  useful  effect  of  the  arrangement  depends,  were 
the  only  action,  the  supply  would  be  perfectly  safe  to  the 


n 

7 

0 

U         A 

rt 

Fio.  55. 

user  so  long  as  apparatus  with  which  he  could  not  interfere 
was  in  proper  order.  But  the  electromagnetic  action  is 
not  the  only  one.  Theoretically  speaking,  every  induction 
coil  is  also  a  condenser,  and  the  primary  coil  acts  electro- 
statically as  well  as  electromagnetically  upon  the  secondary 
coil.  This  electrostatic  action  may  easily  become  danger- 
ous if  the  secondary  generator  is  so  constructed  that  its 
electrostatic  capacity,  regarded  as  a  condenser,  is  other 
than  a  very  small  quantity. 


AN   UNNOTICED   DANGER.  179 

Imagine  an  alternate  current  dynamo  machine,  A,  Fig. 
55,  its  terminals,  B,  C,  connected  by  a  continuous  con- 
ductor, B  D  C,  on  which  may  be  resistances,  self  induction 
coils,  secondary  generators,  or  any  other  appliances  :  at  any 
point  is  a  condenser,  E,  one  coating  of  which  is  connected 
to  the  conductor,  or  may  indeed  be  part  of  it  ;  the  other  is 
connected  to  earth  through  a  resistance,  R.  Let  K  be  the 
capacity  of  the  condenser,  Fthe  potential  at  time  t  of  the 
earth  coating  of  the  condenser,  U  the  potential  of  the  other 
coating,  2  the  current  in  resistance  R  to  the  condenser  from 
the  earth,  being  taken  as  positive,  and  the  earth  potential 
as  zero.  We  have 


, 

whence,  since 

U  =  A  sin  2  n  n  t, 

where  A  is  a  constant  depending  on  the  circumstances  of 
the  dynamo  circuit  as  well  as  the  electromotive  force  of  the 
machine,  and  n  is  the  reciprocal  of  the  periodic  time  of  the 
machine,  we  have 

KR  %  +  x  =  2  n  n  KA  cosZrtn  t, 
9  -  a        {  —  2  7t  n  KR  sin  2  n  n  t  -j-  cos  2  n  n  t  \  , 


x  = 


- 
mean  square  of  x  =  .  mean  square  of  A. 


Let  us  now  consider  the  actual  values  likely  to  occur  in 
practice.     Let  the  condenser  E  be  a  secondary  generator; 


180  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

let  the  resistance  R  be  that  of  some  person  touching  some 
part  of  the  secondary  circuit,  and  also  making  contact  to 
earth  with  some  other  part  of  the  body ;  n  may  be  anything 
from  100  to  250,  say  150;  /Twill  depend  on  the  construc- 
tion of  the  secondary  generator — it  may  be  as  high  as  0.3 
microfarad  or  even  more,  but  there  would  be  no  difficulty 
even  in  large  instruments  in  keeping  it  down  to  one  hun- 
dredth of  this  or  less.  The  mean  square  of  A  will  depend 
on  the  circumstances  of  other  parts  of  the  circuit;  it  might 
very  easily  be  as  great,  or  very  nearly  as  great,  as  the  mean 
difference  of  potential  between  the  terminals  of  the  ma- 
chine if  the  primary  circuit  were  to  earth  at  C.  Suppose, 
however,  that  the  circuit  B  D  C  is  symmetrical,  that  E  is 
at  one  end,  and  that  another  person  of  the  same  resistance 
as  the  person  at  E  is  touching  the  secondary  circuit  of  the 
secondary  generator  F  at  the  other  end  of  the  circuit.  In 
that  case,  if  2,400  be  difference  of  potential  of  the  machine, 
mean  square  of  A  will  be  1,200;  in  which  case  we  have, 
taking  R  as  2,000  ohms, 

mean  square  of 

2  n  X  150  X  0.3  X  10 


,     r. 


==== 

t/(2  n  X  150  X  0.3  X  10-6  X  2,000)'  +  1 

=  about  0.3  ampere. 

Experiments  are  still  wanting  to  show  what  current  may 
be  considered  as  certain  to  kill  a  man,  but  it  is  very  doubtful 
whether  any  man  could  stand  0.3  ampere  for  a  sensible 
length  of  time.  It  is  probable  that  if  the  two  persons  both 
'took  firm  hold  of  the  secondary  conductors  of  E  and  F9 
both  would  be  killed.  If  the  person  at  F  be  replaced  by 


AN  UNNOTICED  DANGER.  181 

an  accidental  dead  earth  on  the  secondary  circuit  of  F,  the 
person  at  E  would  experience  a  greater  current  than  0.3 
ampere. 

It  follows  from  the  preceding  consideration  that  second- 
ary generators  of  large  electrostatic  capacity  are  essentially 
dangerous,  even  though  the  insulation  of  the  primary  cir- 
cuit and  of  the  primary  coils  from  the  secondary  coils  is 
perfect.  The  moral  is — for  the  constructor,  Take  care  that 
the  secondary  generators  have  not  a  large  electrostatic 
capacity,  say  not  more  than  0.03  microfarad,  better  less 
than  -jlnj-  microfarad;  for  the  inspector,  Test  the  system 
for  safety.  The  test  is  very  easy.  Place  a  secondary  gen- 
erator of  greatest  capacity  at  one  end  of  the  line  and  con- 
nect its  secondary  circuit  to  earth  through  any  instrument 
suitable  for  measuring  alternate  currents  under  one  am- 
pere; put  the  other  end  of  the  primary  to  earth;  the  read- 
ing of  the  current  measuring  instrument  should  not  exceed 
such  a  current  as  it  may  be  demonstrated  a  man  can  en- 
dure with  safety. 


182  DYNAMO  MACHINERY   AND  ALLIED   SUBJECTS. 


INDUCTION  COILS  OR  TRANSFORMERS. 

THE  transformers  considered  are  those  having  a  con- 
tinuous iron  magnetic  circuit  of  uniform  section.* 
Let  A  be  area  of  section  of  the  core; 

m  and  u  the  number  of  convolutions  of  the  primary 

and  secondary  coils,  respectively; 
R,  r,  and  p  their  resistances,  p  being  the  resistance 

of  the  secondary  external  to  the  transformer; 
x  and  y  currents  in  the  two  coils; 
a  induction  per  square  centimetre; 
a  the  magnetic  force; 
I  the  length  of  the  magnetic  circuit; 
E  =  B  sin  2  n  (t/T),  the  difference  of  potentials 

between  the  extremities  of  the  primary ; 
T  being  the  periodic  time. 
We  have 

4  it  (m x  +  ny)  =  I  a;  (1) 


E  =  R  x  —  m  A  a; 
0  =  (r  +  p)y-nAa. 

(2) 
(3) 

*  For  a  discussion  of  transformers  in  which  there  is  a  considerable  gap  in  the 
magnetic  circuit,  see  Ferraris,  Torino,  Accad.  Sci.  Mem.,  vol.  37,  1885  ;  also 
chapter  on  the  "  Theory  of  Alternating  Currents,"  in  this  volume, 


INDUCTION   COILS   OR  TRANSFORMERS.  183 

From  (2)  and  (3), 

n  E  =  n  R  x  —  m  (r  +  p)  y.  (4) 

Substituting  from  (1), 

x\n*R  +  m*(r  +  p)\  =  <tf  E  +  (la/±7t)  m(r  +  p);  (5) 
y{n*R  +  m*(r  +  p)}  =-nmE+  (lac/±7t)nR;    (6) 

(r  +  p)mE  laR(r  +  p)  _  ,  , 

n*  7*  +  ra*  (r  +  p)  T  4  Trjrc2  7?  +  ma  (/•  H-  p)  {'  v  ; 

We  may  now  advantageously  make  a  first  approximation. 
Neglect  I  ty  in  comparison  with  knmx,  that  is,  assume  the 
permeability  to  be  very  large;  we  have 


m  .. 

*  * 


~      '  *-' 


For  practical  purposes  these  equations  are  really  suf- 
ficient. 

We  see  first  that  the  transformer  transforms  the  poten- 
tial in  the  ratio  n/vn,  and  adds  to  the  external  resistance 
of  the  secondary  circuit  p  a  resistance  (n*  R/m*}  +  r. 
This  at  once  gives  us  the  variation  of  potential  caused  by 
varying  the  number  of  lamps  used.  The  phase  of  the  sec- 
ondary current  is  exactly  opposite  to  that  of  the  primary. 

In  designing  a  transformer  it  is  particularly  necessary  to 
take  note  of  equation  (9),  for  the  assumption  is  that  a  is 
limited  so  that  I  <x  may  be  neglected,  The  greatest  value 


184   DYNAMO   MACHINERY   AND   ALLIED  SUBJECTS. 

of  a  is  R/{(2  Tt/T) mA\,  and  this  must  not  exceed  a 
chosen  value.  We  observe  that  B  varies  as  the  number  of 
reversals  of  the  primary  current  per  unit  of  time. 

But  this  first  approximation,  though  enough  for  practical 
work,  gives  no  account  of  what  happens  when  transformers 
are  worked  so  that  the  iron  is  nearly  saturated,  or  how 
energy  is  wasted  in  the  iron  core  by  the  continual  reversal 
of  its  magnetism.  The  amount  of  such  waste  is  easily 


Fio.  66. 


estimated  from  Swing's  results  when  the  extreme  value  of 
a  is  known,  but  it  is  more  instructive  to  proceed  to  a  second 
approximation,  and  see  how  the  magnetic  properties  of  the 
iron  affect  the  value  and  phase  of  x  and  y.  We  shall,  as  a 
second  approximation,  substitute  in  equations  (5),  (6),  (7) 


INDUCTION   COILS   OR  TRANSFORMERS.  185 

values  of  a  deduced  from  the  value  of  a  furnished  by  the 
first  approximation  in  equation  (9). 

In  the  accompanying  diagram,  Fig.  56,  Ox  represents  or, 
0  y  represents  «,  and  0  z  the  time  t. 

The  curves  A  B  CD  represent  the  relations  of  a  and  a. 
E  F  G  the  induction  a  as  a  function  of  the  time,  and  HIK 
the  deduced  relation  between  a  and  t.  We  may  substitute 
the  values  of  a  obtained  from  this  curve  in  equations  (5) 
and  (6),  and  so  obtain  the  values  of  x  and  y  to  a  higher 
degree  of  approximation.  If  the  values  of  a  were  expressed 


Fro.  57. 

by  Fourier's  theorem  in  terms  of  the  time,  we  should  find 
that  the  action  of  the  iron  core  introduced  into  the  ex- 
pression for  x  and  y,  in  addition  to  a  term  in  cos  (2  TC  t/T) 
which  would  occur  if  a  and  a  were  proportional,  terms  in 
sin  (2  it  t/T)  and  terms  in  sines  and  cosines  of  multiples 
of  Znt/T.  It  is  through  the  term  in  sin  (%7tt/T)  that 
the  loss  of  energy  by  hysteresis  comes  in. 

A  particular  case,  in  which  to  stay  at  a  first  approxima- 
tion would  be  very  misleading,  is  worthy  of  note.  Let  an 
attempt  be  made  to  ascertain  the  highest  possible  values  of 


186  DYNAMO  MACHINERY  AND  ALLIED   SUBJECTS. 

a  by  using  upon  a  transformer  a  very  large  primary  current 
and  measuring  the  consequent  mean  square  of  potential  in 
the  secondary  circuit  by  means  of  an  electrometer,  by  the 
heating  of  a  conductor,  or  other  such  device.  The  value 
of  a  will  be  related  to  the  time  somewhat  as  indicated  by 
AB  C D  E FO  in  Fig.  57;  for  simplicity  assume  it  to  bo 
as  in  Fig.  58;  the  resulting  relations  of  potential  in  the 


Fio.  88. 

secondary  and  the  time  will  be  indicated  by  the  dotted  line 
HIJKOL  MNP  Q.  The  mean  square  observed  will  be 
proportional  to  ML .  ^L  P ;  but  ML .  L  P  is  proportional 
to  EL,  hence  the  potential  observed  will  vary  inversely  as 
^L  P,  even  though  the  maximum  induction  remain  con- 
stant. If,  then,  the  maximum  induction  be  deduced  on 
the  assumption  that  the  induction  is  a  simple  harmonic 
function  of  the  time,  results  may  readily  be  obtained  vastly 
in  excess  of  the  truth. 


TEST   OF   WESTINGIIOTJSE   TRANSFORMERS.      187 


REPORT    TO   THE    WESTINGHOUSE    COMPANY 
OF  THE  TEST  OF  TWO  G,500-WATT  WEST- 
INGHOUSE TRANSFORMERS. 

BEFORE  giving  any  of  the  results  of  the  tests  I  have 
made  with  your  tran sformers,  it  will  be  well  to  explain  the 
methods  of  experiment  adopted.  The  instantaneous  value 
at  any  epoch  in  the  period  of  the  difference  of  potential 
between  any  two  points  of  a  circuit  in  which  the  potential 
difference  is  varied  periodically  is  made  effective  on  the 
measuring  instrument  by  means  of  a  rotating  contact 
maker  attached  to  the  shaft  of  the  alternate  current  gen- 
erator. This  contact  maker  was  constructed  for  the  King's 
College  laboratory  by  Messrs.  Siemens  Brothers.  It  makes 
contact  once  in  each  revolution  for  a  period  of  about  three 
quarters  of  a  degree,  and  breaks  it  for  the  rest  of  the  revo- 
lution. It  is  entirely  insulated,  and  so  can  be  connected  to 
any  part  of  the  circuit.  The  position  of  the  contact  can 
be  varied,  and  the  variation  be  read  off  on  a  graduated 
circle  of  13J  inches  diameter  divided  into  degrees,  and  by 
estimation  the  variation  can  be  read  to  one  tenth  of  a 
degree.  The  two  points  between  which  it  is  desired  to 
measure  a  potential  difference  are  connected  through  the 
contact  maker  to  a.  condenser  and  a  quadrant  electrometer, 


188    DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

as  shown  in  Fig.  59,  in  which  A  and  B  are  the  points,  the 
potential  difference  of  which  at  a  stated  epoch  is  to  be 
measured,  C  the  revolving  contact  maker,  D  the  reversing 
switch  of  the  electrometer,  E  the  condenser,  of  which  the 
capacity  can  be  varied,  F  the  quadrant  electrometer.  It  is 
evident  that  the  quadrant  electrometer  will  give  a  reading 
proportional  to  the  potential  difference  of  A  and  B,  when 
C  makes  contact.  If  there  were  no  leakage,  it  would  at 


once  give  this  potential.  It  is  to  obviate  the  effect  of 
leakage  that  the  condenser  is  introduced,  and  the  amount 
of  the  effect  was  determined  by  varying  the  condenser 
thus:  When  the  condenser  had  capacity  1,  0.5,  and  0.2 
microfarads,  the  readings  of  the  electrometer  for  a  given 
potential  difference  of  an  alternating  current  at  the  posi- 
tion in  the  period  of  maximum  electromotive  force  were 
138,  136,  and  132,  respectively.  The  rate  of  loss  of  poten- 
tial will  be  proportional  to  the  reciprocal  of  the  capacity, 
whence  we  infer  that  the  true  reading,  if  insulation  were 
perfect,  would  be  1394,  and  hence  the  readings  are  always 
corrected  by  adding  one  per  cent.  When  the  potential 
difference  was  too  great  for  the  electrometer  it  was  reduced 
in  any  desired  ratio  by  two  considerable  resistances  intro- 
duced between  the  points  to  be  measured  in  the  usual  way 
(Fig.  60).  The  potential  difference  may,  of  course,  be 
measured  in  other  ways.  An  ordinary  voltmeter  may  be 


TEST  OF  WESTINGHOUSE  TRANSFORMERS.    189 

placed  between  A  and  B,  in  which  case  it  must  be  standard- 
ized with  the  contact  breaker  in  circuit;  and  it  will  depend 
for  its  constant  on  the  duration  of  the  contact,  which  may 
vary.  Further,  it  gives,  not  the  difference  of  potential 


Fio.  60. 

at  any  definite  epoch,  but  the  mean  difference  for  the 
whole  time  of  the  contact.  The  condenser  may  be  used 
and  its  potential  be  measured  by  discharge  through  a  gal- 
vanometer: this  is  open  to  the  objection  that  if  there  be 
any  leakage,  the  result  will  depend  on  the  time  at  which 
the  contact  is  broken  by  the  condenser  key  in  relation  to 
the  time  at  which  it  was  made  by  the  revolving  contact 
maker.  Lastly,  a  Clark  cell  may  be  used,  by  a  method 
which  Major  Cardew  pointed  out  to  me  (Fig.  61),  the  re- 


iDJUSTABLt  RESISTANCES  '          (]   GALVANO 

rmmwms — QC     V 


FlO.  61. 


sistance  being  adjusted  till  there  is  no  deflection.  This 
is  open  to  the  same  objection  as  the  first,  namely,  that  it 
gives  the  mean  of  the  potential  differences  which  occur 
during  the  contact.  By  making  use  of  the  first-mentioned 
method  we  have  the  means  of  measuring  accurately  any 
potential  difference  at  any  epoch  of  the  period,  and  of 
knowing  the  epoch. 


190  DYNAMO   MACHINERY  AND  ALLIED   SUBJECTS. 

For  these  experiments  two  transformers  intended  to  be 
identical  were  available,  each  transforming  between  2,400 
and  100  volts.  It  was  most  convenient  on  account  of  the 
resistance  available  to  couple  these"  transformers  up  from 
100  to  2,400  in  the  first  or  No.  1  transformer,  then  down 
from  2,400  to  100  in  the  second  or  No.  2  transformer,  and 
to  take  up  the  energy  from  the  second  in  a  non-inductive 
resistance.  The  arrangement  is  shown  in  Fig.  62. 

The  obvious  way  in  returning  the  efficiency  of  the  com- 
bination would  be  to  measure,  at  various  epochs  of  half  a 
period,  the  potential  differences  of  the  terminals  of  the 


TRANSFORMED  NO.  1.    TRANSFORMER  NO. 2. 

FM.OL 

machine  and  the  current  passing  in  the  No.  1  transformer; 
in  like  manner,  at  the  same  epochs,  to  measure  either  the 
potential  differences  or  the  current  passing  to  the  non-in- 
ductive resistance,  thence  to  deduce  the  power  supplied  to 
the  first  transformer  and  taken  from  the  second.  This 
would  be  open  to  certain  objections:  we  are  comparing 
two  nearly  equal  magnitudes  and  desire  their  ratio;  the 
ratio  will  be  afflicted  with  the  full  error  arising  from  an 
error  in  the  determination  of  either  magnitude,  and  such 
errors  may  be  material,  as  the  observations  are  not  simul- 
taneous, and  conditions  may  change  between  one  series  of 
observations  and  another. 

These  objections  are   avoided  by  the   method  adopted. 
The  current  from  No.  2  is  observed  at  certain  epochs,  the 


TEST   OF    WESTINGIIOTJSE  TfcANSFOttlVlEfcS.      191 

difference  of  current  between  No.  2  and  No.  1,  and  the 
difference  of  potential  difference  of  No.  2  and  No.  1  at  the 
same  epoch.  These  give  the  currents  and  potentials  of 
No.  1  at  the  same  epochs  as  the  corresponding  determina- 
tions of  No.  2,  and  the  difference  will  only  be  afflicted  with 
the  proportion  of  error  of  those  differences.  For  example, 
suppose  the  efficiency  of  the  combination  were  90  per  cent., 
and  the  possible  error  of  determination  of  power  1  per 
cent.,  our  result  might  be  anything  from  38  per  cent,  to  92 
per  cent,  if  made  in  the  obvious  way,  but  if  made  by  dif- 
ferences the  maximum  loss  would  be  10.1  per  cent.,  and 
the  possible  least  determination  of  the  efficiency  would  be 
89.8  per  cent.  The  method  is  essentially  similar  to  the 


>TO  NON-INDUCTIVE 
*       RESISTANCE 


NO. 2. 1 

TO  REVOLVING  CONTACT  MAKER 
FIG.  63. 


method  I  described*  and  subsequently  used  for  testing 
dynamos.  The  measurements  for  difference  of  potential 
differences  are  made  as  in  Fig.  63.  For  current  differences 
(Fig.  64),  where  G  is  a  known  small  non-inductive  resist- 


*TO  NON-INDUCTIVE 


TO  REVOLVING  CONTACT  MAKER 
FIG.  64. 


ance,  the  two  currents  will,  of  course,  slightly  disturb  each 
other,  but  this  is  readily  allowed  for  in  the  calculations. 

*  Phil.  Trans.,  1886,  page  347. 


192  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

Another  method  would  be  to  couple  them  as  in  Fig. 
65,  Gv  and  #,  being  equal  non-inductive  resistances. 
This  arrangement  is  quite  free  from  disturbance,  but  re- 
quires two  resistances  adjusted  to  exact  equality.  A  single 
transformer  can  be  tested  in  the  same  way,  though  in  this 


1 

TO  REVOLVING  CONTACT'MAKER 
FIG.  65. 


case  reliance  must  be  placed  upon  resistances  to  reduce  the 
current  of  the  low  potential  coil,  and  to  reduce  the  poten- 
tial of  the  high  potential  coil  in  the  ratio  of  the  number  of 
windings  in  the  two  coils. 

The  current  was  throughout  generated  by  a  Siemens 
alternator  with  12  magnets,  run  at  a  speed  between  830 
and  840  revolutions  per  minute,  which  gives  a  frequency 
of  5,000  per  minute,  or  83  to  84  per  second. 

The  first  experiment  tried  *  was  with  the  two  transform- 
ers coupled,  but  with  No.  2  transformer  on  open  circuit,  or 
on  nearly  open  circuit,  for  a  high  resistance  for  purposes 
of  measurement  was  interposed  between  the  terminals  of 
the  low  resistance  coil  of  No.  2  transformer.  The  actual 


*  So  far  as  I  know,  the  first  discussion  of  endless  magnetic  circuit  transform- 
ers, based  on  the  actual  properties  of  the  material,  is  in  a  note  by  myself  (Proc. 
Roy.  Soc.,  vol.  xui.,  and  Tiie  Electrician,  vol.  xvni.,  p.  421.)  Definite  results 
were  obtained  by  methods  generally  similar  to  those  now  used  by  Prof.  Ryan 
(The  Electrical  World,  Dec.  28,  1889).  The  theory  of  transformers  is  well  set 
forth  by  Prof.  Fleming  (The  Electrician,  April  22  and  29,  1892). 


TEST   OF    WESTINGHOUSE  TRANSFORMERS.      193 


results  are  given  in  Table  IX.,  and  are  expressed  in  Fig.  66. 
Tables  X.,  XL,  and  XII.  give  the  results  for  half  power, 

f 


TABLE  IX. 


S3 

Potential  No.  2. 

Potential  No.  1. 

n 

Thick  Coils. 

Thick  Coils. 

s° 
S§. 

m 

g«o 

No.  1. 
Thick 
Coils. 

Amperes. 

Square  of 
Volts. 
Vmean2 
=  101.9. 

Watts 
supplied  to 
No.  1. 

Volts. 

Square  of 
Volts. 

P.  D. 

Sec  Nos. 
1  and  2. 

Volts. 

Vineau3 

=  101.1 

Volts. 

267 

-2.2 

+  25.4 

645 

-0.9 

+  26.3 

692 

-    57.9 

270 

-0.3 

+  70.2 

4,9:28 

-1.2 

+  71.4 

5,098 

-    21.4 

273 

4-1.1 

+  95.3 

9,082 

-1.1 

+  96.4 

9,292 

+  106.0 

276 

+  2.1 

+120.4 

14,496 

-1.1 

--121.5 

14,761 

+  255.1 

279 

+  2.8 

+147.7 

21,816 

-1.1 

--148.8 

22,140 

+  416.6 

282 

285 

+  3.2 
+  3.4 

+147.2 
+119.8 

21,668 
14,351 

+  0.9 

+  0.7 

+148.1 
--120.5 

21,935 
14,520 

+  473.9 
+  409.7 

288 
291 
294 

+  3.5 
+  3.7 
+  3.5 

+  97.8 
+  26io 

9,565 
5,084 
676 

+  0.6 
+  0.4 
+  0.3 

+  98.4 
--  71.7 
--  25.97 

9,683 
5,140 
674 

+  344.4 
+  260.3 
+    90.9 

102,311 

103,935 

2,282.6 

194  DYNAMO   MACHINERY   AND   ALLIED 

nearly  full  power,  and  full  power,  and  the  sets  of  curves  of 
Figs.  67,  68  and  69  give  the  results  of  the  table.  In 
these  tables  the  first  column  gives  the  position  of  the  con- 
tact brush  in  degrees,  so  that  60  on  this  scale  corresponds 

with  a  complete  cycle.     Three  degrees  are  thus  -^-^ — 

.  Oo.o   X    <v(J 

of  a  second.  The  second  column  of  Table  IX.  is  the  cur- 
rent in  the  thick  coil  of  No.  1  transformer,  as  determined 
by  the  difference  of  potential  at  the  two  ends  of  a  non- 
inductive  resistance  in  which  the  current  passes.  The 
third  column  is  the  potential  difference  of  No.  2  trans- 
former, a  direct  determination.  The  fourth  column  is 
solely  for  the  purpose  of  determining  the  square  root  of 
the  mean  of  the  squares  of  the  third  column.  This  column 
is  a  direct  determination  of  the  difference  of  potential  of 
No.  1  and  No.  2,  obtained  in  the  manner  explained  with 
reference  to  Fig.  63. 

The  sixth  column  is  the  deduced  potential  difference  of 
the  terminals  of  the  thick  wire  of  No.  1  transformer,  being 
the  sum  of  the  third  and  fifth  columns.  The  seventh  col- 
umn, like  the  fourth,  is  merely  for  the  purpose  of  deter- 
mining the  square  root  of  the  mean  of  the  squares  of  col- 
umn six,  while  the  eighth  gives  the  rate  at  which  power  is 
given  out  or  received  by  the  pair  of  transformers. 

If  the  transformers  had  been  exactly  equal,  the  poten- 
tials for  the  two  given  by  Table  IX.  would  have  been  equal, 
though  they  would  have  differed  a  little  in  phase  owing  to 
the  lines  of  magnetic  induction  which  pass  through  the 
non-magnetic  space  between  the  two  coils  of  the  trans- 
former.*  The  difference  shows  that  No.  1  transformer  has 

*Prof.  Perry  has  already  pointed  out  that  the  effect  of  such  an  induction 
cannot  be  entirely  neglected,  even  in  endless  circuit  transformers. 


TEST   OF   WESTINGHOUSE  TRANSFORMERS.      195 

a  ratio  of  transformation  slightly  greater  than  No.  2.     If 
we  correct  the  potential  of  either  No.  1  or  No.  2,  there 


196  DYNAMO   MACHINEKY   AND   ALLIED   SUBJECTS. 


still  remains  a  difference  between  them,  but  this  difference 
will  be  greatest  about  when  the  potentials  are  nil.    This 


is  due  to  the  lost  induction  just  referred  to.    In  order  to 
check  the  conclusion  that  the  two  transformers  are  not 


KHOM  WAyHTNCtERMINXtB 


NAAA/WV \^A/Wv* 

ywvwvwvww /WW.VWWM 

TO  CONTACT  MAMER  iEl.ECTROME.1ER, 

Fio.70 


precisely  equal,  they  were  directly  compared,  as  in  Fig.  70. 
The  transformers  were  coupled  parallel,  as  in  Fig.  70,  and 


TEST   OF   WESTINGHOUSE   TRANSFORMERS.       197 


the  difference  of  potential  of  the  two  high  potential  coils 
was  measured :  the  value  of  its  square  root  of  mean  square 
was  12.5  volts,  the  potential  of  the  transformer  being  2,400. 
This  does  not  necessarily  imply  that  the  potentials  of  the 
two  transformers  differ  by  one  half  per  cent.;  it  maybe 
largely  due  to  a  difference  of  phase  between  the  two. 

The  current  supplied  to  the  No.  1  transformer  is  to  be 
accounted  for  by  the  currents  necessary  to  magnetize  the 


Z 


FIG.  71. 


two  transformers,  and  by  the  local  currents  in  their  cores. 
To  ascertain  the  former,  the  curve  of  magnetization  of  one 
of  the  transformers  was  determined  by  the  ballistic  galva- 
nometer for  nearly  the  same  induction  as  in  Table  IX.,  the 
changes  of  current  supplied  by  a  battery  being  made  by  a 
reversing  switch,  or  by  suddenly  introducing  resistance 


198   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

into  the  primary  circuit,  and  the  consequent  changes  of 
induction  being  measured  by  the  galvanometer.  The 
tardiness  of  change  of  current  in  the  transformer  due  to 
its  self  induction  was  sufficiently  reduced  by  using  many 
cells  and  a  considerable  resistance.  The  results  are  shown 
in  Fig.  71  for  a  single  transformer.  In  this  curve  the 
abscissae  are  the  currents  in  the  thin  coil  of  the  trans- 


former,  divided  by  24  to  reduce  it  to  the  same  effect  as  it 
would  have  had  if  it  had  been  in  the  thick  coil.  The 
ordinates  are  the  inductions  as  measured  by  the  kick  on 
the  galvanometer,  but  reduced  to  a  scale  to  make  them 
directly  comparable  with  the  volts  when  the  transformer  is 
used  with  an  alternating  current.  These  results  are  not 


TEST   OF   WESTINGHOUSE   TRANSFORMERS.      199 

given  in  absolute  units.  The  procedure  to  determine 
points  on  the  curve  was:  first,  pass  the  maximum  current 
corresponding  to  the  point  C\  next,  suddenly  diminish  the 
current  by  inserting  suitable  resistance  in  the  thin  coil 
circuit,  and  observe  the  kick — the  drop  of  ordinate  from 
C  to  A  corresponds  to  the  kick,  and  the  abscissa  of  A  is 
the  current  after  it  has  been  reduced;  next,  reverse  the 
current,  and  observe  the  kick — the  kick  corresponds  to  the 
further  drop  of  ordinates  from  A  to  B.  In  this  manner  a 
series  of  points  are  determined  on  the  curve.  Fig.  72 
shows  the  relation  between  induction  and  magnetizing 
current  for  the  pair  of  transformers,  as  deduced  from  the 
experiments  with  alternating  currents  set  forth  in  Table  IX. 
The  ordinates  in  this  curve  are  the  area  of  the  curve  of 
potentials  of  Fig.  66,  for  the  ordinates  of  this  latter  curve 
are  the  rates  at  which  the  induction  is  changing,  while  the 
abscissae  are  the  currents  in  the  thick  wire  at  corresponding 
times.  The  points  marked  •  in  Fig.  73  give  the  remainder 
after  deducting  the  magnetizing  current  as  estimates  in 
Fig.  71  from  the  currents  of  Fig.  72 — that  is  to  say,  Fig. 
71  is  corrected  first  for  the  small  difference  in  maximum 
induction;  then,  corresponding  to  any  induction,  the  cur- 
rent is  taken  from  the  curve,  it  is  doubled,  as  there  is  only 
one  transformer,  and  the  result  is  deduced  from  the  cor- 
responding current  of  Fig.  72.  The  differences  are,  the 
magnetizing  current  equivalent  and  opposite  in  effect  to 
the  local  currents  in  the  cores.  If  the  local  currents  were 
equivalent  to  a  current  in  a  single  secondary  circuit,  the 
points  •  of  Fig.  73  ought  to  have  had  the  form  of  the  full 
line  of  Fig.  73,  drawn  through  the  points  -f,  in  which  the 
abscissae  are  proportional  to  the  potential  difference,  and 


200   DYNAMO    MACHINERY    AND   ALLIED    SUBJECTS. 

the  ordinates  to  the  induction.  Returning  to  Table  IX.,  we 
find  that  the  fall  of  potential  difference  on  open  circuit  in 
the  whole  combination  is  0.8  volt,  and  that  the  loss  of 


power  in  magnetizing  the  cores  and  in  local  currents  is 
222.86  watts,  that  is,  a  loss  for  each  transformer  of  114.13 
watts.  The  total  loss  of  228  watts  maybe  divided  into  126 
watts  accounted  for  by  hysteresis  and  102  watts  due  to 
local  currents. 

Referring  now  to  Table  XI.  and  Fig.  68,  the  earlier  col- 
umns explain  themselves,  but  a  word  is  necessary  about 
the  last  six  columns.  The  watts  supplied  to  No.  1  are 
simply  the  products  at  each  time  of  the  volts  at  its  termi- 


TEST   OF    WESTINGHOUSE   TRANSFORMERS.       201 

nals  and  the  amperes  passing  through,  similarly  to  the 
watts  given  out  by  No.  2.  We  see  first  that  the  efficiency 
of  the  whole  combination  with  this  load  is  93.73  per  cent., 
and  hence  the  efficiency  of  one  transformer,  if  the  losses 
in  the  two  are  equal,  may  be  taken  as  96.9  per  cent.  The 
fall  of  potential  in  the  whole  combination  is  6.1  volts,  but 
the  fall  with  no  load  is  0.8  volt;  hence  the  variation  due  to 
the  load  with  constant  potential  on  the  thin  coil  of  No.  1 
is  5.3  volts,  or,  if  the  fall  of  potential  in  the  two  transform- 
ers were  equal,  which  it  is  not,  for  a  single  transformer 
2.65  volts.  Assuming  that  the  transformers  are  equal,  the 
power  lost  in  resistance  would  be  expected  to  be  the  mean 
of  mean  current  X  the  difference  of  potential  difference, 
or  215.4  watts.  It  is,  in  fact,  150  watts,  as  given  by  multi- 
plying the  square  of  currents  by  resistances.  But  the 
transformers  are  not  exactly  equal,  and  there  is  the  waste 
magnetic  field,  both  of  which  will  have  a  small  effect  on 
the  distribution  of  loss  between  the  two  classes  of  loss, — 
viz.,  that  by  hysteresis  and  local  currents,  and  that  by  re- 
sistance,— but  none  upon  the  gross  efficiency. 

The  other  tables,  X.  and  XII.,  are  arranged  in  exactly  the 
same  way  as  Table  XL,  but  the  number  of  observations 
on  Table  XII.  is  insufficient  to  bring  out  all  the  peculiari- 
ties of  the  transformers. 

It  has  already  been  stated  that,  if  the  loss  of  potential 
due  to  load  in  the  two  transformers  be  equal,  it  will  amount 
to  2.65  per  cent.  The  following  experiment  was  tried  to 
ascertain  if  this  loss  was  equal:  The  transformers  were 
coupled  in  series  as  before.  The  mean  potential  difference 
of  the  thick  wire  was  measured  by  Thomson's  multicellu- 
lar,  and  of  the  thin  wire  by  Thomson's  electrostatic  volt- 


202   DYNAMO    MACHINERY   AND    ALLIED   SUBJECTS. 


meter.  The  mean  of  a  considerable  number  of  experiments 
is  given  in  the  following  table,  the  load  being  the  same  as 
in  Table  XL,  and  the  results  being  corrected  to  the  same 
potential  of  the  thin  wire : — 


Number. 

Full  Load. 

Open  Circuit. 

Thomson's 
Multicellular. 

Thomson's 
Electrostatic. 

Thomson's 
Multicellular. 

Thomson's 
Electrostatic. 

1 
2 

2,380 
2,380 

99.8 
94.2 

2,380 
2,380 

97.0 
96.2 

This  shows  that  of  a  total  drop  of  4.8  volts,  2.8  volts  oc- 
curred in  No.  1,  and  2  volts  in  No.  2.  There  is  no  doubt 
of  the  fact  that  the  drop  is  greater  in  No.  1  than  in  No.  2, 
which  is  connected  with  the  waste  field  between  the  two 
coils.  Of  course  these  transformers  are  intended  to  work 
exactly  as  No.  2  is  working,  in  which  case  the  drop  from 
no  load  to  nearly  full  load,  as  shown  by  this  experiment,  is 
2.0  volts.  The  way  in  which  this  waste  field  causes  ine- 
quality of  drop  of  potential  in  the  two  transformers,  coupled 
as  in  my  experiments,  is  well  worthy  of  careful  considera- 
tion. The  waste  field  is  proportional  to  the  current  in  the 
transformers,  or,  better,  to  the  mean  of  the  two  currents  in 
ampere  turns.  The  electromotive  force  due  to  this  waste 
field  will  be  proportional  to  the  rate  of  change  of  the  cur- 
rent. If  the  current  were  expressed  by  a  simple  harmonic 

curve,  the  electromotive  force  due  to  the  waste  field  would 

jyr 
also  be  a  simple  harmonic  curve  differing  in  phase  by  — . 

The  curve  of  potentials  is  roughly  in  the  same  phase  as  the 


TEST   OF    WESTINGHOUSE   TRANSFORMERS.       203 

curve  of  current.  Let  A  be  the  amplitude  of  potential 
difference  of  No.  2  transformer,  B  be  the  amplitude  of 
difference  of  potential  difference  in  No.  2,  or  the  potential 
difference  of  the  thin  wire  divided  by  24.  2  b  will  be  very 
nearly  the  amplitude  of  difference  between  the  thick  wires 
o'f  Nos.  1  and  2.  The  ratios  of  potentials  in  No.  1  and 
No.  2  will  then  be 


Va?  +  b9 


and 


2  a 


a   and  1 


or  the  drop  in  the  first  from  this  cause  is  three  times  as 
great  as  in  the  second  transformer.  We  shall  return  to  the 
waste  field  immediately.  Putting  aside  harmonic  curves, 
and  returning  to  the  facts  as  they  are,  the  following  table 
gives:  first,  half  the  difference  of  potential  difference 


Half  Difference  of  Potential 
Difference. 

Volts  of  High  Potential 
Coil  Divided  by  24. 

Squares  of  Volts. 

|                  15.6 

-5.0 

25.0 

148 

41.3 

1,706.0 

11.3 

74.3 

5,520.0 

10.8 

102.6 

10,530.0 

11.1 

131.3 

17,240.0 

5.5 

147.1 

21,640.0 

—  1.1 

137.0 

18,770.0 

-  3.1 

119.3 

14,230.0 

-  5.9 

95.1 

9,040.0' 

-12.1 

53.4 

2,850.0 

Square  root  of  mean  square  =  100.8. 


taken  from  Table  XI.,  that  is,  at  each  instant  the  drop  of 
potential  in  No.  2;  secondly,  the  volts  of  the  thin  wire  of 
No,  2  reduced  for  number  of  convolutions — this  is  of  course 


204   DYNAMO   MACHINERY   AND    ALLIED   SUBJECTS. 

the  mean  of  potential  difference  between  1  and  2 ;  lastly, 
the  squares  of  these  volts.  From  this  we  see  a  mean  square 
100.8  showing  the  drop  in  No.  2  to  be  2.6  volts  out  of  a 
total  drop  of  6.1,  and  the  remainder  2.5,  the  drop  in  No.  1. 
Diminishing  these  results  by  0.4,  the  half  of  0.8,  the  fall 
observed  with  no  load,  the  actual  losses  from  no  load  to 
nearly  full  load  will  be  2.2  and  3.1. 

Turn  now  to  the  last  column  of  Table  XI.  This  gives 
the  difference  of  potential  differences  corrected  for  the  loss 
of  volts  by  resistance.  It  is  shown  dotted  on  Fig.  68;  this 
curve  presents  one  or  two  peculiar  features.  It  should  be 
possible  to  infer  the  form  of  this  curve  from  the  curve  of 
current.  The  rates  at  which  the  mean  current  is  changing 
are  as  follows : 

268*  271*  274*  277*  280*  283*  286*  289*  292*  .... 
30.7  24.2  19.2  18.5  18.9  1.7  -9.8  -12.7  -21.7  -28.1 

—which  happens  to  come  to  a  scale  which  can  be  at  once 
plotted.  The  points  marked  •  are  the  points  of  the  curve 
corresponding  with  the  above  rates.  The  agreement  of 
the  points  with  the  curve  is  remarkably  close.  This  ex- 
hibits very  completely  the  effect  of  waste  magnetic  field  in 
this  transformer. 

For  half  power,  as  taken  from  Table  X.,  the  rates  are  as 
follows : 

268*6  271*  274*  277*  280*  283*  286*  289*  291>*  295* 
14.6  11.5  9.4  10.6  4.4  -4.6  -7.2  -7.5  -13.6  -17.6 

— and  in  the  same  way  in  Fig.  G7  the  dotted  curve  represents 
the  difference  of  electromotive  force  corrected  for  resist- 
ance, and  the  points  correspond  with  the  above  rates. 


TEST   OF   WESTINGHOTJSE  TRANSFORMERS.      205 


Fig.  74  gives  the  efficiencies  for  the  combined  transform- 
ers in  terms  of  the  load.     This  curve  is  the  hyperbole : 


Efficiency  =  100 . 


where  A  =  228,  the  loss  by  hysteresis; 

B   =  0.005,  and   mainly  depends    upon  the  waste 

field; 

C   =  0.0000035,  and  is   mainly  the  loss  by  resist- 
ance; 

X   =  load  in  watts. 

To  sum  up,  I  find  that  the  efficiency  of  the  transformer 
at  full  load  would  be  96.9  per  cent. ;  at  half  load,  96  per 

JOOi — 


Load  in  103  watte. 
Fio.  74. 

cent.;  and  at  quarter  load,  over  92  per  cent.  The  magnet- 
izing current  of  the  transformer  amounts  to  114  watts,  or 
1.75  per  cent.  The  drop  of  potential  from  no  load  to  full 
load  is  between  2  per  cent,  and  2.2  per  cent. 

In  conclusion,  I  wish  to  express  my  thanks  to  Mr.  Wil- 
son, of  King's  College;  this  gentleman  carried  out  the 
experiments  under  my  direction,  and  made  nearly  all  the 
numerical  calculations  and  drew  most  of  the  curves  for  me. 


DYNAMO   MACIIINEKY   AND   ALLIED  SUBJECTS. 


x 


ential  No. 
Thick  Coil. 


Pot 


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ST   OF   WESTINGilOUSE  TRANSFORMERS.      CJ07 


luiiuejoj  jo  8ou8jajji(i 


putt  sisaaajsA'H 


ss  by  Res 
and  pro 
Waste 


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DYNAMO  MACHINERY  AND   ALLIED   SUBJECT* 


l«ooq  pins  8;s<u9isA'H  A'q  asoq 


Loss  by 
ance  and 
ably  Wast 


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tential  Diffe 
ence  No  1. 
Thick  Coil. 


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TEST   OF   WESTINGHOUSE   TRANSFORMERS.      209 


Potential  No.  2. 
Thick  Coil. 

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•sil°  A  jo  Qjtmbs 

rp      £1      Q      Q      eg        a 

98893     S 

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•*       •«*       •*       00       Qt 

S2    S    8    S  •  S 

+  +   +   +  + 

Current  No.  1.  Thick  Coil. 

•saj^dtay 

«o     o>     to     e*     GO 

a  s  s  «  a 

+  +  +  +  + 

Current  Difference.  Thick  Co.  Is. 
Nos.  1  and  2. 

•s.u^duiv 

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1      +    +    +    + 

> 

1-1          TH          0>          1>          1-1 

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+      1       1       1       1 

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p»joajjoo 

|  fe  §  te  | 

•uonoatpd 
paAJdsqo 

fe    fe    ^    38    X 

^H                          ^          ^-H          O| 

Current  No.  2. 
Thick  Coil. 

•saaedrav 

00       O       CO       CO       0 

S    S    S    S    5 

+  -f  +  +  + 

•uowoagaa  ps^oaojoo 

gw     o     os     «o 
8     5!     S     8 

•uonoayaa  pQAjasqo 

SCO         f>         i-l         t-. 
O       C*       00       O 
<?i        ^S*        CO        d 

"IPO  papiAta 
no  qsrug  Saijoidxa  jo  sp«aq 

O       O       C*l       CO       ^* 

S    K    8    V    i 

210  DYNAMO  MACHINKRY   AND   ALLIED  SUBJECTS. 


s  «  s 

+   i    i 


^-.     co      «o 


iwooq  pav  sjs^jaja.f  H  ^q  ssoq 


2 


88    g 

+   +   +   -K 


J5f2' 

i 


O»       «O       O       CO 


'aoj 
pe?unooovun    sassoq 


i 


+   Hh   +     1 


00       0       09       CO 

8  i  S  • 


t- 

i 


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!    !  ! 


Is 


put?  i  -BOM  ai  n?nu»joa  jo  UTOK 


8         S 


« 


ntial  Differe 
Thick  Coils. 
Nos.  1  and  2. 


ei     10     o»     co     «o 

8    §    2    S    S 

+    +   7    Hh   + 


§5  s;  s  »  s 
+  +  +    1    1 


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i  (<n.i  ;  |  >uijo|'  I  x  [4  jo  spvi 


g  S  1 


THEORY  OF  THE  ALTERNATE  CURRENT  DYNAMO. 

\ 


THEORY  OF  THE  ALTERNATE  CURRENT 
DYNAMO. 

ACCORDING  to  the  accepted  theory  of  the  alternate  current 
dynamo,  the  equation  of  electric  current  in  the  armature 
is  y  y  +  R  y  —  periodic  function  of  t,  where  y  is  a  constant 
coefficient  of  self  induction.  This  equation  is  not  strictly 
true,  inasmuch  as  y  is  not  in  general  constant,*  but  it  is  a 
most  useful  approximation.  My  present  purpose  is  to  in- 
dicate how  the  values  of  y  and  of  the  periodic  function 
representing  the  electromotive  force  can  be  calculated  in  a 
machine  of  given  configuration. 

To  fix  ideas,  we  will  suppose  the  machine  considered  to 
have  its  magnet  cores  arranged  parallel  to  the  axis  of  rota- 
tion, that  the  cores  are  of  uniform  section,  also  that  the 
armature  bobbins  have  iron  cores,  so  that  we  regard  all  the 
lines  of  induction  as  passing  either  through  an  armature 
coil  or  else  between  adjacent  poles  entirely  outside  the 
armature.  The  sketch,  Fig.  75,  shows  a  development  of 
the  machine  considered.  The  iron  is  supposed  to  be  so 
arranged  that  the  currents  induced  therein  may  be  neg- 
lected. We  further  suppose  for  simplicity  that  the  line 
integral  of  magnetic  force  within  the  armature  core  may 
be  neglected. 

*  See  chapter  on  the  "  Theory  of  Alternating  Currents"  in  this  volume. 


212  DYNAMO  MACHINERY  AND   ALLIED  SUBJECTS. 
i 

Let  At  be  the  effective  area  of  the  space  between  the  pole 
piece  and  armature  core  when  the  cores  are  in  line,  J,  the 
distance  from  iron  to  iron. 

Let  A9  be  the  section  of  magnet  core,  7,  the  effective 
length  of  a  pair  of  magnet  limbs,  so  that  /,  may  be  re- 


Fro.  76. 

garded  as  the  length  of  the  lines  of  force  as  measured  from 
one  pole  face  to  the  next. 

Let  m  be  the  number  of  convolutions  in  a  pair  of  magnet 
limbs,  and 

n  the  convolutions  in  one  armature  section; 

T  the  periodic  time. 

The  time  is  measured  from  an  epoch  when  the  armature 
coil  we  shall  consider  is  in  a  symmetrical  position  in  a  field 
which  we  shall  regard  as  positive. 

x  and  y  are  the  currents  in  the  magnet  and  armature 
coils,  the  positive  direction  being  that  which  produces  the 
positive  field  at  time  zero. 

At  time  t  the  armature  coil  considered  has  area  Af, 

=  ft0  +  ft,  cos  (2  n  t/T)  +  ft,  cos  (4  it  t/T)  +  etc., 


THEORY  OF  THE  ALTERNATE  CURRENT  DYNAMO.    213 
in  a  positive  field;  and  area  A"9 

=  b0-b,  cos  (2  n  t/T)  +  b,  cos  (4  n  t/T)  -  etc., 
in  a  negative  field,  where 


and 

&o  ~  &,  +  t>,  +   -  -  -   =  0. 

The  coefficients  b0,  b},  etc.,  are  deducible  by  Fourier's 
theorem  from  a  drawing  of  the  machine  under  considera- 
tion. 

Let  /  be  the  total  induction  in  the  magnet  core,  and  let, 
at  time  t,  /be  distributed  into  /'  through  A'9  I"  through 
A"  and  /'"  as  a  waste  field  to  the  neighboring  poles. 

The  line  integral  of  magnetic  force  from  the  pole  to 
either  adjacent  pole  is  /'"/&,  where  k  is  a  constant. 

We  have  first  to  determine  /',  1",  /'",  in  terms  of  x 
and  y. 

Take  the  line  integral  of  magnetic  force  in  three  ways 
through  the  magnets,  and  respectively  through  area  A', 
through  area  A",  and  across  between  the  adjacent  poles— 


f  +  3  l  '    ~'    =  4  *  m  X  ~~  4  *  H  y 


214  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 
whence 


A,'  -A 


When  t,  x  and  y  are  given,  this  would  suffice  to  determine 
1  by  means  of  the  known  properties  of  the  material  of  the 
magnets  as  represented  by  the  function  /.  We  will,  how- 
ever, consider  two  extreme  cases  between  which  other  cases 
will  lie. 

First.  Suppose  that  the  intensity  of  induction  in  the 
magnet  cores  is  small,  so  that  /,/(//^4a)  may  be  neglected, 
the  iron  being  very  far  from  saturation.  We  have 


=  2T!m  (A'  "  A")  x  +  n-  (A*  +  A 

±7t    (         (.  27Tt     .  67Tt  \ 

=  -y-  1  m  \tt  cos  -y-  -f  J,  cos  -^  +  ...) 
+  n     0  -f  b,  cos  --  +  .  .  . 


We  see  that  the  coefficient  of  self  induction  y  in  general 
contains  terms  in  cos  (4  n  t/T). 
Second,  In  actual  work  it  would  be  nearer  the  truth  to 


THEORY  OF  THE  ALTERNATE  CURRENT  DYNAMO.     215 

suppose  that  the  magnetizing  current  x  is  so  great  that  the 
induction  /  may  be  regarded  as  constant,  and  the  quantity 
l^f(I/A^  as  considerable.  But  as  small  changes  in  / 
imply  very  great  changes  in  l^f(I/A^),  its  value  cannot  be 
regarded  as  known.  We  have  then 


(A* A 
I--1  2l 


A/ -A" 
- 


whence 

r    r,=    (A; -A,") i 
(A:  -  A,")' 


(A,'  -  A,")  I 
~  A^  +  AS'  +  Zkl, 

4  A,'  A,"  +  2 k I,  (A,'  +  A,")  titny 
'          "  •  • 


\ 
216   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

For  illustration,  consider  the  simplest  possible  case:  let 
b0  =  b.  =  $At  ,  and  b9  =  ft,  =  .  .  .  =  0,  and  let  2  k  Z,  be 
negligible;  we  have 

T/,  2jrtf,  .2  TT  t  47rny 

I  -  I    =  Jcos  -y-  +  Al  sin9  —^-  .  -y^, 

and  the  equation  of  current  will  be 


instead  of  the  simple  and  familiar  linear  equation. 


THE  ELECTRIC   LIGHTHOUSE  OF  MACQUAKIE.    217 


THE  ELECTRIC  LIGHTHOUSES  OF  MACQUARIE 
AND  OF  TINO. 

THE  subject  of  the  use  of  the  electric  light  in  light- 
houses was  fully  discussed  at  the  Institution  in  1879,  when 
papers  by  Sir  James  Douglass,  M.  Inst.  C.E.,  and  by  Mr. 
James  T.  Chance,  Asso.  Inst.  C.E.,  were  read.* 

The  subject  has  been  further  elaborately  examined  by 
Mr.  E.  Allard,f  and  more  recently  in  practical  experi- 
ments made  at  the  South  Foreland,  exhaustively  reported 
on  by  a  committee  of  the  Trinity  House.J  The  justifica- 
tion of  the  present  communication  is  that,  at  the  light- 
houses of  Macquarie  and  of  Tino,  the  optical  apparatus  is  on 
a  larger  scale  than  has  hitherto  been  used  for  the  electric 
arc  in  .lighthouses,  and  presents  certain  novel  features  in 
the  details  of  construction.  Further,  as  regards  the  elec- 
trical apparatus,  tests  were  made  upon  the  machinery  for 
Macquarie  when  it  was  in  the  hands  of  Messrs.  Chance 
Brothers  &  Company,  which  still  possess  some  value, 
although  five  years  old;  and,  in  the  case  of  Tino,  the 
machines  are  practically  worked  together  in  a  manner 
not  previously  used  otherwise  than  by  way  of  experiment. 

*  Minutes  of  Proceedings  of  the  Inst.  C.E.,  vol.  LVII.,  pp.  77  and  168. 
t  "  M6moire  sur  leg  Phares  filectriques,"  1881. 

%  "  Report  into  the  relative  merits  of  Electricity,  Gas,  and  Oil  as  Lighthouse 
Illuminants."  Parts  1  and  2,  PP.  1885, 


218   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

In  the  case  of  both  lighthouses,  Messrs.  Chance  Brothers 
&  Company,  of  Birmingham,  entered  into  a  contract  for 
the  supply  of  all  the  apparatus  required,  including  engines, 
machines,  conductors,  lamps,  optical  apparatus,  and  lan- 
terns; and  Sir  James  Douglass,  engineer-in-chief  of  the 
Trinity  House,  acted  as  inspecting  engineer  to  the  respec- 
tive colonial  and  foreign  governments. 

As  these  two  lighthouses  present  many  features  in  com- 
mon, it  may  be  most  convenient  to  give  a  full  description 
of  the  earlier  lighthouse,  and  then  limit  the  description  of 
Tino  to  those  points  in  which  it  differs  from  Macquarie. 


MACQUARIE. 

This  lighthouse  is  situated  on  South  Head,  near  Sydney,  , 
the  precise  position  being  shown  in  a  copy  from  the 
chart,  Fig.  76.  A  lighthouse  was  first  placed  at  this  im- 
portant landfall  in  1817.  The  focal  plane  is  346  feet 
above  the  sea,  and  the  distance  of  the  sea  horizon  is  there- 
fore 21.6  nautical  miles,  and  the  range  about  27  nautical 
miles  for  an  observer  1 5  feet  above  the  sea. 

Optical  Apparatus. — The  light  is  a  revolving  one,  giv- 
ing a  single  flash  of  eight  seconds'  duration  every  minute. 
On  account  of  the  considerable  altitude  of  the  lighthouse, 
it  was  necessary  to  secure  that  a  substantial  quantity  of 
light  should  be  directed  to  the  nearer  sea;  but  it  was  also 
essential,  on  account  of  the  exceptional  power  of  the  ap- 
paratus, that  this  dipping  light  should  only  be  a  small 
fraction  of  that  sent  to  the  horizon,  otherwise  its  effect 
would  be  excessively  dazzling.  Many  years  ago  Mr, 


THE 


ELECTRIC  LIGHTHOUSE  OF  MACQUARIE.  219 


EAST  MAITLAND   JN. 

?/     A  NEWCASTLE 


FIG.  7<J. 


220   DYNAMO   MACHINERY   AND   ALLIED    SUBJECTS. 

James  T.  Chance  urged  that  it  was  not  wise  to  make  use 
of  very  small  apparatus  for  the  electric  arc,  because  a 
larger  apparatus  renders  it  possible  for  the  optical  engineer 
to  effect  with  greater  precision  the  distribution  of  light 
which  is  most  desirable,  and  because  any  trifling  error 
which  may  occur  in  the  position  of  the  electric  arc  lias, 
with  the  larger  apparatus,  a  less  marked  effect  on  the  light 
as  seen  from  the  sea.  In  the  lighthouses  of  Souter  Point, 
the  South  Foreland,  and  the  Lizard,  the  third  order  ap- 
paratus of  500  millimetre  focal  length  was  adopted. 
Optically,  the  larger  the  apparatus  used  the  better,  but 
there  might  be  some  question  whether,  on  purely  optical 
grounds,  the  advantage  of  going  beyond  the  third  order  is 
sufficient  to  justify  the  additional  expense;  but  in  the  case 
of  a  revolving  apparatus  the  third  order  is  a  very  incon- 
venient size  for  the  service  of  the  lamp — it  is  too  large  to 
be  conveniently  served  from  the  outside,  and  too  small  to 
admit  the  attendant  within  it  with  comfort.  With  the 
large  currents,  which  are  now  easily  obtained  and  are 
likely  to  be  used  in  lighthouses,  a  first  or  second  order 
apparatus  has  the  further  advantage  that  it  is  less  liable  to 
injury  from  particles  thrown  off  from  the  heated  carbons. 
In  the  case  of  Macquarie,  it  was  decided  to  adopt  an  ap- 
paratus of  the  first  order,  920  millimetre  focal  length;  it 
was  further  decided  that  the  optical  apparatus  should  pro- 
duce its  condensing  effect  by  means  of  a  single  agent — that 
is  to  say,  the  vertical  straight  prisms  which  were  used  in 
Souter  Point  and  other  revolving  electric  lighthouses 
should  be  dispensed  with.  The  condensation  and  dis- 
tribution of  light  necessary  may  be  obtained  by  means  of 
a  single  agent,  with  apparatus  such  as  has  been  pro- 


THE   ELECTRIC   LIGHTHOUSE   OF   MACQCJARLE. 

posed  by  Mr.  Alan  Brebner,  Jr.,  Asso.  M.  Inst.  O.E.;* 
but  this  construction  is  open  to  the  objections  that  it 
is  somewhat  costly,  and  that  it  increases  the  length  of 
the  path  of  the  rays  through  the  glass,  and  consequent 
absorption.  A  practically  better  plan  is  to  adopt  forms 
not  differing  very  greatly  from  those  introduced  by 
Fresnel;  to  specially  arrange  them  for  the  purpose  in 
hand,  and  to  accept  certain  consequent  minute  deviations 
from  a  mathematically  accurate  solution  for  the  sake  of 
advantages  of  greater  importance  when  all  the  actual  con- 
ditions are  taken  into  account.  Fig.  77  shows  the  optical 
apparatus  in  vertical  section:  the  upper  and  the  lower 
totally  reflecting  prisms  are,  as  is  usual  in  revolving  lights, 
forms  of  revolutions  about  a  horizontal  axis;  they  direct 
the  light  incident  upon  them  to  the  horizon  and  the  dis- 
tant sea  from  10'  above  the  horizon  to  30'  below;  they  are 
specially  adjusted  to  distribute  the  light  in  azimuth  over 
the  arc  of  3°  necessary  for  a  proper  duration  of  flash. 

The  refracting  portion  of  the  apparatus  has  the  profile 
so  calculated  that  the  central  lens  and  the  three  rings 
next  to  the  lens  above  and  below  direct  their  light  to  the 
horizon  without  vertical  divergence,  except  what  is  due  to 
the  size  of  the  arc.  The  light  for  the  nearer  sea  is  obtained 
from  the  remaining  ten  lens  segments,  Nos.  5  to  9  in- 
clusive, above  and  below  the  centre,  counting  the  centre  as 
No.  1,  the  distribution  being  according  to  the  following 
table,  in  which  the  first  column  gives  the  denomination*  of 
the  elements  of  the  lens  in  accordance  with  the  numbers 
marked  upon  the  section ;  the  second,  the  angle  between 


*  Minutes  of  Proceedings  of  the  Inst.  C.E.,  vol.  LXX.,  p.  386. 


222   DYNAMO  MACHINERY  AND   ALLIED  SUBJECTS. 
L  II.  HI. 

O          I          It  O  t  II 

gtop —10  0  2    30    59 

8  »    2    30  59  5      8    52 

7  "    —10  0  2    87    30 

6  ••   —10  0  1    30      0 

5  "    ....  —10  0  100 

Sbottom..  —10  0  100 

6  "  —10  0  1  30  0 

7  "  1    80  0  3  44  27 

8  •«  3    44  27  5  50  41 

9  "  5    50  41  7  46  57 

the  direction  of  the  sea  horizon  and  the  ray  emerging  from 
the  upper  limit  of  the  element;  the  third,  the  angle  be- 
tween the  direction  of  the  sea  horizon  and  the  ray  from 
the  lower  limit  of  the  element,  the  negative  sign  denoting 
that  the  emerging  ray  is  above  the  horizon.  This  practice 
of  appropriating  certain  elements  of  the  apparatus  to  dif- 
ferent distances  on  the  sea  was  first  introduced  by  Mr. 
James  T.  Chance,  in  the  lights  of  the  South  Foreland  ex- 
hibited in  January,  1872. 

The  ray,  dipping  at  an  angle  of  7°  46'  57'  below  the 
horizon,  will  strike  the  sea  at  £  mile,  while  5°  8'  52'  cor- 
responds to  }  mile,  2°  37'  30'  to  1±  mile,  1°  30'  to  2  miles, 
1°  to  2i  miles,  and  30'  to  about  4  miles.  Thus  the  direct 
light  begins  at  about  i  mile  from  the  lighthouse.  From  | 
mile  to  |  mile  the  sea  receives  light  from  one  element  of 
the  apparatus,  from  J  to  1£  mile  from  two  elements,  from 
1J  mile  to  2  miles  from  three  elements,  from  2  to  2£  miles 
from  four  elements,  and  beyond  2£  miles  from  six 
elements;  the  upper  and  lower  totally  reflecting  prisms 
come  in  aid  at  about  5  miles.  The  main  power  of  the  ap- 
paratus is  hardly  attained  till  a  distance  of  8  or  10  miles. 
Fig.  78  is  a  sectional  plan  of  the  apparatus  by  a  horizontal 


THE  ELECTRIC  LIGHTHOUSE  OF  MACQITARIE. 
Fio.  77.  Fia.  79. 


FIG.  78. 


FIG.  80. 


224  DYNAMO  MACHINERY  AND   ALLIED  SUBJECTS. 

plane  through  the  focus.  It  will  be  seen  that  a  dioptric 
mirror  is  placed  on  the  landward  side  of  the  arc.  This 
mirror  is  arranged  to  form  the  image  of  the  arc  at  one 
side  of  the  carbons,  so  avoiding  the  interception  of  light 
which  would  result  if  the  mirror  were  used  in  the  ordinary 
way,  and  contributing  to  the  horizontal  divergence  neces- 
sary. Further  horizontal  divergence  is  given  by  the  form 
of  the  lens.  In  the  ordinary  revolving  light  the  inner  face 
of  the  lens  is  plane;  here  it  is  cylindrical,  the  axis  of  the 
cylinder  being  vertical.  This  method  of  obtaining  hori- 
zontal divergence  is  a  modification  of  a  proposal  of  Mr. 
Thomas  Stevenson,*  M.  Insl.  C.E.;  it  is  not  mathemati- 
cally accurate,  inasmuch  as  the  cylindrical  form  of  the 
inner  face  of  the  lens  not  only  displaces  the  emergent  ray 
horizontally,  but  also,  in  the  case  of  rays  not  in  the  verti- 
cal nor  horizontal  plane  through  the  focus,  to  a  small  ex- 
tent vertically;  but  the  error  is  easily  calculable,  and  is 
unimportant,  provided  the  lens  is  narrow,  and  the  hori- 
zontal divergence  of  the  beam  moderate.  Fig.  79  shows  a 
complete  panel  in  elevation  with  revolving  carriage.  Fig. 
80  shows  the  plan  of  the  service  table  of  the  pedestal  and 
lamp  table.  A  new  construction  was  adopted  for  the  gnn- 
metal  framework  of  the  optical  apparatus  to  reduce  the 
interception  of  light  by  the  frame  to  a  minimum.  The 
metal  segment  A,  Fig.  81,  forms  part  of  the  lower  prism 
frame,  B  part  of  the  upper  frame,  while  C  and  D  are  parts 
of  the  frame  for  the  refracting  portion  of  the  apparatus; 
uprights  E  support  the  upper  prism  frames  without  throw- 
ing weight  on  the  lens  frames.  With  the  ordinary  con- 

*"  Lighthouse  Construction  and  Illumination,"  p.  186. 


THE   ELECTRIC   LIGHTHOUSE   OF   MACQUARIE.    225 


structions  of  frame,  Figs.  82  and  83,  the  equivalent  of  these 
ring  segments  A  and  B  would  intercept  about  double  as 
much  light  as  in  this  new  construction. 


E 


Fio.  81. 


FIG  82. 


Fio.  83. 


Mechanism  for  Rotation. — The  pedestal  is  similar  to 
those  designed  by  Sir  James  Douglass  to  permit  the  light 
keeper  to  obtain  access  from  below  to  the  interior  of  the 
apparatus  without  in  any  way  interfering  with  its  rotation. 
The  clockwork  is  fitted  with  the  governor,  and  maintain- 
ing poweV  used  by  Messrs.  Chance  Brothers  &  Company 
for  the  last  twelve  years.  The  roller  ring  may  be  men- 
tioned as  of  an  improved  type,  for  although  it  has  been 
used  for  some  years  in  all  Messrs.  Chance's  lights,  it  has 
not  been  described  before.  The  rollers  and  roller  paths 
which  carry  the  whole  weight  of  the  optical  apparatus 
have  long  been  made  conical,  so  that  the  surfaces  roll 


226   DYNAMO   MACHINERY   AND   ALLIED   SUfeJECTS. 

upon  each  other  without  twisting.  There  is  consequently 
a  very  considerable  radial  force  on  each  roller  tending  to 
force  it  outwards;  the  reaction  against  this  force  causes  a 
very  important  part  of  the  total  frictional  resistance.  Fig. 
84  shows  a  portion  of  the  roller  ring  and  one  of  the  conical 


Fio.  84.  Fio.  86.  FIG.  86. 

rollers,  according  to  the  old  construction;  Figs.  85,  86,  ac- 
cording to  the  improved  construction;  in  the  former  it 
will  be  observed  that  the  thrust  of  the  roller  is  received  on 
a  collar;  in  the  latter,  on  the  end  of  a  pin.  The  reduction 
of  friction  is  practically  very  considerable,  and  although  of 
small  importance  in  a  slow-moving  apparatus  like  Mac- 
quarie,  is  of  great  importance  in  heavier  and  quicker  ap- 
paratus; for  example,  the  triple  flashing  light  at  Bull 
Point,  in  Devonshire. 

Lamp*.— These  are  of  the  Serrin  type,  and  were  supplied 
by  Baron  De  Meritens. 

Lamp  Table. — The  arrangements  for  rapidly  changing 
electric  lamps,  and  for  substituting  gas  or  oil  when  de- 
sired, are  shown  in  Figs.  87,  88,  89,  and  90. 

The  intention  was  to  use  a  gas  lamp  in  clear  weather, 
and  half  power  or  full  power  electric  light  in  thick 
weather,  according  to  the  opacity  of  the  atmosphere;  but 
the  author  understands  that  in  practice  the  electric  arc  is 


THE   ELECTRIC   LIGHTHOUSE   OF   MACQUARIE.    227 


always  used.     The  paraffin  oil  lamp  is  intended  as  a  re- 
source in  case  of  failure  of  the  supply  of  gas. 


FIG.  87. 


Fio.  88. 


FIG.  89. 
ARRANGEMENT  FOR  OIL  LAMP. 


FIG.  90. 
ARRANGEMENT  FOR  GAS  BURNER. 


Focussing   the    Arc. — Two    approximately  rectangular 
prisms  are  fixed  upon  the  mirrcr  frame  at  about  90°  from 


228   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

each  other,  the  longer  face  of  each  is  plane,  the  other  two 
faces  convex,  of  such  curvature  as  to  form  a  good  image 
of  the  arc  upon  the  service  table,  as  shown  in  Fig.  91. 


Fio.  91. 

During  daylight,  a  pointed  sight  or  focimeter  is  placed  at 
the  position  of  the  image  formed  by  the  lens  of  an  object 
on  the  horizon;  this  then  is  the  position  which  the  arc 
should  occupy.  A  sight  is  next  taken  over  the  focimeter 
into  one  of  the  adjusting  prisms,  and  a  bright  object  such 
as  a  threepenny  piece  placed  on  the  service  table,  is  moved 
about  until  its  centre  is  seen  in  the  prism,  exactly  upon 
the  point  of  the  focimeter;  a  mark  is  made  in  the  then 
position  of  the  object.  When  the  arc  is  corrootly  adjusted, 
its  image  on  the  service  table  will  be  at  the  point  where 
the  mark  is  made.  Two  prisms  are  used  in  order  to  secure 
that  the  arc  shall  be  in  the  centre  of  the  apparatus  as  well 
as  at  the  correct  level. 

Lantern. — The  lantern  is  of  the  well  known  Douglass 
type.* 

*  Minutes  of  Proceedings  of  the  List.  C.E.,  vol.  LVI.,  p.  77. 


THE   ELECTRIC   LIGHTHOUSE   OF   MACQUAKIE.    229 

Dynamo-Electric  Machines. — Two  alternate  current  ma- 
chines, with  permanent  magnets  manufactured  by  De 
Meritens,  were  supplied.  Each  machine  has  five  rings  in 
its  armature,  and  in  each  ring  there  are  sixteen  segments. 
In  supplying  one  arc  for  a  lighthouse  the  machine  runs 
about  830  revolutions  per  minute,  and  gives  a  current  of 
55  amperes  when  half  the  coils  are  used,  and  of  110  when 
the  whole  of  the  machine  is  in  action,  the  internal  resist- 
ance in  the  two  cases  being  0.062  and  0.031  ohm.  It  is 
unnecessary  to  give  a  description  of  the  machine  as  its 
general  construction  and  dimensions  are  well  known,  but 
some  numerical  details  are  given  below. 

Engines. — Each  machine  is  driven  by  an  8  h.  p.  Crossley 
gas  engine  through  a  belt  without  countershafting. 

Tests. — Whilst  the  dynamo  machines  were  at  the  works 
of  Messrs.  Chance  Brothers  &  Company,  a  series  of  ex- 
periments was  made  in  March,  1881,  to  determine  their 
properties.  The  time  is  long  passed  when  it  would  be 
profitable  to  give  the  details  of  these  experiments,  but  the 
general  conclusions  drawn  at  the  time  are  still  interesting. 
When  the  external  resistance  was  a  metallic  conductor 
with  small  self  induction,  it  was  found  that  with  varying 
resistance  and  speed  the  currents  observed  agreed  fairly 


well  with  calculation  from  the  formula 


in  which  R  is  the  total  resistance  of  the  circuit,  y  the  self 
induction,  and  T  the  periodic  time.     When  the  machine 

*  Lectures  on  the  "Practical  Application  of  Electricity.1'    Session  1883-83. 
Paper  on  "  Some  Points  in  Electric  Lighting,"  reprinted  in  this  volume. 


230  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 
was  running  830  revolutions  per  minute  A  =  67  volts  and 

=  0-19?   i11    onm    s<i"aiv«l,   hence  y  =  6.4  x  10* 


centimetres.  The  eighty  sections  of  the  machine  ;t  it- 
arranged  four  in  series,  twenty  parallel.  For  a  single  sec- 
tion the  value  of  y  would  be  32  x  10*  centimetres.  The 
maximum  induction  in  the  core,  which  has  an  area  of  5 
square  centimetres,  is  24,600  or  4,920  per  square  centi- 
metre. The  loss  of  power  was  greater  when  the  machine 
was  doing  little  or  no  external  work  than  when  that  work 
was  great.  This  is  clearly  seen  in  the  following  table  :— 

Current  amperes  ...............................     ........  7.70  73.  GO 

Electrical  work  h.  p  ...................................  0.69  5.66 

Mechanical  work  applied  ...............................  8.09  6.56 

Loss  .................................................  2.40  0.89 

Photometric  experiments  were  made  upon  the  arc,  and 
simultaneous  measurements  of  effective  power  applied  and 
of  current  passing.  The  red  light  was  measured  through 
bright  copper  ruby  glass,  and  the  blue  through  a  solution 
of  sulphate  of  copper  and  ammonia.  The  h.  p.  was  meas- 
ured by  a  transmission  dynamometer;  but  the  results  must 
be  accepted  with  some  reserve,  on  account  of  the  difficulty 
of  ascertaining  the  mean  tension  in  a  strap  which  is  con- 
stantly varying.  The  oscillations  of  the  dynamometer 
were  damped  by  a  dashpot  containing  tar. 

Half  Power.  Full  Power. 
Redcandles  ........................................     1,988  I.'"* 

Blue     "        ..........................................    4,079  11,382 

Current  (amperes)  ....................................    54.5  105 

Mechanical  power  applied  (h.  p.)  ..................      4.5  6.9 

Power  expended  in  heating  conducting  wires  (h,  p.)     0.34  0.95 


THE   ELECTRIC   LIGHTHOUSE   OF   TINO.  231 

The  results  illustrate  the  fact  that,  as  the  current  in- 
creases, the  total  light  increases  in  a  higher  ratio,  red 
light  in  a  slightly  higher  ratio,  and  blue  in  a  considerably 
higher. 

The  machinery  for  this  lighthouse  was  sent  out  to  New 
South  Wales  in  November,  1881,  and  was  put  up  and 
started  under  the  superintendence  of  Mr.  J.  Burnett,  the 
architect  of  the  colony,  to  whom  is  mainly  due  the  success 
of  the  whole  from  the  first  start.  The  glare  of  the  light 
upon  the  sky  is  said  to  have  been  seen  at  a  distance  of 
over  GO  miles,  far  beyond  the  distance  at  which  it  would 
cease  to  be  directly  visible.  The  only  criticism  from 
mariners  has  been  that  when  somewhat  near  the  light- 
house the  flashes  are  so  bright  as  to  dazzle  the  eye.  This 
is  an  excellent  proof  of  the  power  of  the  light,  as  a  much 
smaller  proportion  of  the  light  is  directed  upon  the  nearer 
sea  than  in  any  previous  lighthouse.  The  lesson  is  that 
with  powerful  electric  lighthouses  almost  all  the  light 
should,  in  ordinary  weather  at  least,  be  directed  to  the 
horizon,  and  that  the  quantity  thrown  upon  the  nearer  sea 
must  be  strictly  limited.  This  is  only  possible  when  the 
focal  length  of  the  apparatus  is  large. 


TINO. 

This  station  is  on  a  small  island  at  the  mouth  of  the 
Gulf  of  Spezia.  Fig.  92  is  copied  from  the  chart  of  the 
neighborhood.  The  focus  is  386  feet  above  sea  level.  The 
distance  of  the  sea  horizon  is  22.7  nautical  miles,  and 
the  range  practically  28  miles.  The  conditions,  therefore, 


232    DYNAMO   MACHINERY    AND   ALLIED   SUBJECTS. 

were  very  similar  to  those  of  Macquarie,  with  the  excep- 
tion that  it  was  required  to  throw  some  light  dowii  into 
the  channel  between  Palmaria  and  Tino.  The  lighthouse 
itself  presents  some  interesting  historical  features.  The 
buildings  were  originally  a  place  of  defence  against  the 
pirates  who  occasionally  made  descents  upon  the  coast. 


Fio.  92.-(Scalet  22  miles  -  j  inch.) 


Subsequently  a  coal  fire  lighthouse  was  established,  and  in 
the  spring  of  1885  part  of  the  stock  of  lignite  was  still 
found  to  be  in  some  of  the  buildings,  where  it  had  been 
lying  for  fifty  years.  In  1839  a  dioptric  light  was  estab- 
lished, one  of  the  earliest  of  Fresnel's  types,  the  lens  ring 
being  replaced  by  short  straight  prisms,  which  formed  by 
no  means  a  bad  approximation,  and  could  be  ground  with- 
out special  machinery.  The  present  electric  lighthouse  has 
been  in  contemplation  for  several  years. 


THE   ELECTRIC   LIGHTHOUSE   OF  TINO.          233 

Optical  Apparatus. — The  distinctive  character  of  the 
light  is  a  triple  flash  every  half  minute.  The  apparatus  for 
producing  this  effect  is  of  the  general  form  introduced  by 
the  author  in  1874.  In  October  of  that  year  he  issued  a 
pamphlet  pointing  out  the  several  advantages  of  group 
flashing  lights,  showing  for  the  first  time  a  simple  dioptric 
apparatus  suitable  to  their  production,  and  also  pointing 
out  how  easy  it  is  to  give  the  group  flashing  effect  with 
catoptric  apparatus.  Since  that  time  a  large  number  of 
dioptric  group  flashing  lights  have  been  made  by  Messrs. 
Chance  Brothers  &  Company,  and  some  also  in  France, 
and  Mr.  Allard  has  incorporated  group  flashing  lights  in 
the  system  of  distinctions  he  recommends ;  also  a  consider- 
able proportion  of  the  light  vessels  on  the  English  coasts 
have  been  converted  into  group  flashing  lights  of  the 
catoptric  system.  On  the  ground  of  economy  the  second 
order  apparatus  of  700  millimetre  focus  was  adopted  in  the 
case  of  Tino.  It  is  just  large  enough  for  tolerably  con- 
venient service  of  the  lamp  by  an  attendant  entering  with- 
in the  apparatus.  The  apparatus,  shown  in  vertical 
section  in  Fig.  93,  and  in  horizontal  section  through  the 
focus  in  Fig.  94,  has  twenty-four  sides,  eight  groups  of 
three;  one  group  of  three  is  shown  in  elevation  in  Fig.  95. 
The  horizontal  divergence  is  obtained  in  exactly  the  same 
way  as  at  Macquarie,  excepting  that  no  mirror  is  used. 
The  metal  framework,  however,  approximates  to  the 
ordinary  type,  as  the  type  used  at  Macquarie  would  have 
been  costly  when  applied  to  a  triple  flash  light.  The  dis- 
tribution of  light  vertically  is  as  follows :  upper  and  lower 
prisms,  and  the  central  lens,  with  the  two  lens  rings  next 
adjoining  it,  all  to  the  horizon  and  most  distant  sea.  The 


234  DYNAMO   MACHINERY  AND  ALLIED*  SUBJECTS. 


Fio.  94. 


FIG.  93. 


THE   ELECTRIC   LIGHTHOUSE   OF   TINO.  235 

lens  and  lens  rings  direct  their  rays  according  to  the  fol- 
lowing table,  which  is  arranged  in  exactly  the  same  way  as 
the  table  already  given  for  Macquarie: — 

i.  n.  in. 


7  top 0    31  35  3    16    0 

6    "    ..  200 

5    "    ..  1    30    0 

4    "     ..  100 

4  bottom ..  0    45    0 

5  "       .  ..  0    30    0 

6  "       ..  0    30    0 

7  "       all  to  the  horizon. 

No.  7  bottom  was  directed  wholly  to  the  horizon,  in 
order  to  avoid  the  horizontal  bar  of  the  lantern.  It  will 
be  observed  that  the  quantity  of  light  thrown  upon  the 
nearer  sea  is  much  less  in  the  case  of  Tino  than  in  that  of 
Macquarie,  and  that  greater  reliance  is  placed  upon  the 
accuracy  with  which  the  arc  can  be  kept  in  focus;  ex- 
perience has  justified  these  changes,  as  improvements  of  a 
perfectly  safe  nature. 

A  small  part  of  each  flash  is  bent  downwards  and  dis- 
tributed over  the  channel  between  Tino  and  Palmaria,  by 
means  of  subsidiary  prisms  fixed  upon  the  lantern,  shown 
at  X,  Fig.  93.  These  subsidiary  prisms  are  really  super- 
fluous, as  the  scattered  light  from  the  beams  overhead  is 
found  to  be  as  effective  at  this  short  distance.  Fig.  90 
shows  the  plan  of  lamp  shunting  table  and  service  table. 

Engines. — As  there  is  no  water  upon  the  island,  the 
practice  of  the  Trinity  House  was  followed,  and  two  of 
the  Brown  hot  air  engines  were  supplied,  each  driving 
through  a  countershaft  one  of  the  machines.  The 
countershafts  could  be  connected  by  means  of  a  Mather 


236    DYNAMO    MACHINERY   AND   ALLIED   SUBJECTS. 

and  Platt  friction  coupling,  so  that  the  two  machines 
could  be  driven  together,  or  either  machine  from  either 
engine.  Drawings  of  the  Brown  engines  are  given  in  Sir 


Fio.  96. 

James  Douglass's  Paper.*  The  accompanying  indicator 
diagrams  Figs.  97  and  98.were  taken  from  the  compressing 
and  working  cylinders.  Whilst  these  diagrams  were  taken, 
the  effective  power  developed  was  measured  by  a  friction 
brake  on  the  driving  pulley,  and  was  found  to  be  9.1  h.  p. 
Thus  of  33.1  h.  p.  indicated  in  the  working  cylinder,  17.7 
h.  p.  is  employed  in  compressing  the  air,  6.3  h.  p.  is  wasted 
in  friction  in  various  parts  of  the  machine,  and  only  9.1 
h.  p.  is  effective  upon  the  brake.  The  engines  consume 
about  4  Ibs.  of  coke  per  effective  h.  p.  per  hour.  In  future 
lighthouses,  when  a  steam  engine  cannot  be  employed,  it 
would  be  preferable  on  every  ground  to  use  gas  engines, 
and  manufacture  on  the  spot  either  Dowson  gas  or 

*  Minutes  of  Proceedings  of  the  Inst.  C.E.,  vol.  LVII.,  Plate  6. 


THE   ELECTRIC   LIGHTHOUSE   OF   TINO. 


237 


ordinary  gas,  according  to  the  character  of  the  fuel  avail- 
able. 

Dynamo  Machines. — There  are  two  machines  of  exactly 


FIG.  97.— Compressor- pump  cylinder,  24  inches  in  diameter.    Stroke,  22  inches. 
Indicated  H.  P.,  17.7. 


Fm.  98.-Workinpr  cylinder.  3'.'  inches  in  diameter.  Stroke,  20  inches.  Indicated 
H.  P.,  33.1.  Revolutions  per  minute,  64.  Power  on  brake  of  fly-wheel,  9.12  H.P. 
Pressure  in  reservoir,  19  to  24  Ibs. 

the  same  type  as  those  supplied  for  Macquarie,  the  only 
novelty  lying  in  the  method  of  using  them.  In  1868  Mr. 
Wilde  discovered,  by  experiment,  that  two  alternate 


238  DYNAMO   MACHINERY   AND  ALLIED  SUBJECTS. 

current  dynamos,  independently  driven  at  the  same  speed, 
would,  if  electrically  connected,  so  control  each  other's 
motions  that  they  would  add  their  currents.  The  author 
subsequently  arrived  at  the  same  conclusion  independently, 
on  theoretical  grounds,  and  gave  a  thorough  explanation  of 
the  fact.*  The  result  has  been  put  to  a  practical  applica- 
tion at  Tino.  The  machines  are  connected  to  a  single 
switchboard,  so  that  each  half  of  the  two  machines  can  at 
pleasure  be  connected  to,  or  disconnected  from,  the  main 
conductors.  Thus  a  current  can  be  supplied  from  either 
machine  at  half  power,  55  amperes,  or  full  power,  110 
amperes,  or  from  the  two  machines  of  double  power,  or 
about  200  amperes.  Further,  a  change  can  be  made  with- 
out extinction  of  the  light  from  one  dynamo  and  engine  to 
the  other.  Thus,  suppose  one  machine  is  working  full 
power,  clutch  the  countershafts  gradually  together,  so 
starting  the  second  engine;  throw  on  the  band  of  the 
second  machine,  cut  out  half  the  first  machine,  and  con- 
nect half  the  second  machine  at  the  switchboard;  the  two 
machines  at  once  synchronize,  without  affecting  the  light. 
Disconnect  the  remaining  half  of  the  first  machine,  and 
connect  the  remaining  half  of  the  second,  unclutch  the 
countershafts,  and  stop  the  first  engine.  One  man  can 
effect  the  change,  with  no  more  disturbance  of  the  light 
than  a  change  from  full  to  half  power  for  about  one 
second.  A  further  conclusion,  deduced  from  theoretical 
considerations,  was  that  of  two  alternate  current  machines 


*  Lectures  on  the  "Practical  Applications  of  Electricity."  Session  1882-88. 
Paper  on  "Some  Points  in  Electric  Lighting,"  reprinted  in  this  volume.  By 
Dr.  John  Hopkinson.  And  Journal  of  the  Society  of  Telegraph  Engineers  and 
Electricians,  vol.  xni.,  p.  496. 


THE  ELECTKtC   LIGHTHOUSE  OF  TINO. 

of  equal  potential,  one  coujd  be  used  as  a  generator  of 
electricity,  the  other  as  a  motor  converting  the  current 
generated  back  into  mechanical  power.  It  was  found 
impossible  to  verify  this  conclusion  with  such  intermittent 
driving  as  that  of  a  hot  air  engine.  But  Professor  W.  G. 
Adams  effected  the  verification  without  difficulty  at  the 
South  Foreland,  the  motive  power  being  steam. 

Lamps. — These  are  the  improved  Serrin  of  Mr.  Berjot. 
One  of  the  three  lamps  supplied  is  of  larger  size,  for  the 
double  power  current  from  the  two  machines.  This  lamp 
was  said  to  be  suitable  for  a  still  greater  current,  but  with 
about  200  amperes  it  soon  became  dangerously  heated;  a 
simple  modification  rendered  the  lamp  equal  to  the  actual 
work  it  had  to  do.  It  is,  however,  probable  that  for  the 
occasional  circumstances  when  it  is  necessary  to  use  so 
great  a  current  as  200  amperes  in  a  lighthouse,  a  lamp 
worked  partly  by  hand  would  be  preferable  to  a  regulator 
entirely  automatic. 

The  apparatus  was  delivered  in  November,  1884,  and 
was  put  up  by  workmen  from  Messrs.  Chance's  workshops, 
under  the  supervision  of  Mr.  L.  Luiggi,  of  Genoa,  to  whose 
ability  and  energy  the  complete  success  of  the  lighthouse 
is  largely  due.  A  complete  test  of  the  performance  of  the 
light,  as  seen  from  the  sea  in  all  grades  of  its  power,  was 
made  in  April,  1885,  by  a  commission,  consisting  of  Profes- 
sor Garibaldi,  of  Genoa;  Mr.  Giaccone,  engineer-in-chief 
for  Italian  lighthouses;  Captain  Sartoris,  and  Mr.  Luiggi, 
the  author  attending  on  behalf  of  Messrs.  Chance.  The 
light  was  well  observed  through  rain,  when  distant  32 
nautical  miles,  and  although  below  the  horizon,  the  posi- 
tion was  precisely  localized,  and  the  triple  flash  distinction 


240  DYNAMO  MACHINERY   AND   ALLIED   SUBJECTS. 

unmistakable.  At  18  miles  distant  the  illumination  of  the 
flash  upon  white  paper  was  sufficient  to  make  out  letters 
marked  in  pencil  l£  inch  high,  and  when  14  miles  distant 
it  was  easy  to  ascertain  the  time  from  a  watch.  The  light 
is  frequently  seen  at  a  distance  of  50  miles,  near  to  Genoa. 

A  review  of  work  which  has  been  carried  out  naturally 
suggests  many  questions  as  to  what  conclusions  experience 
has  established,  and  what  indications  it  gives  of  the  prob- 
able direction  for  future  developments.  In  the  use  of 
electric  light  in  lighthouses,  there  are  many  questions 
upon  which  there  is  wide  difference  of  opinion,  questions 
both  as  to  when  and  where  electric  light  should  be 
adopted,  and  questions  as  to  the  best  way  of  employing  it. 
It  may  not  be  unprofitable  to  allude  to  some  of  them. 
Although  English  engineers  are  now  well  agreed  that  a 
large  optical  apparatus  should  be  used  for  the  electric 
light,  this  opinion  is  not  universally  accepted.  The 
advantages  of  a  large  apparatus  have  already  been  men- 
tioned. To  balance  them,  there  is  nothing  on  the  other 
side  but  the  less  prime  cost  of  the  smaller  apparatus. 
Although  the  difference  of  cost  appears  considerable  when 
attention  is  confined  to  the  optical  apparatus,  it  is  un- 
important when  the  whole  outlay  on  the  lighthouse  is 
brought  into  account.  Cases  are,  however,  conceivable  in 
which  a  small  optical  apparatus  such  as  a  fourth  order, 
having  a  focal  distance  of  250  millimetres,  would  be  prop- 
erly preferred ;  such,  for  example,  as  a  harbor  light  which 
could  be  supplied  with  current  from  machinery  also  used 
for  other  purposes,  but  such  cases  are  likely  to  be  excep- 
tional. 


THE  ELECTRIC   LIGHTHOUSE   OF  TINO.          241 

When  a  flame  from  oil  or  gas  is  the  source  of  light,  there 
is  of  necessity  a  considerable  divergence  vertically  ;  and  the 
distribution  of  the  light  through  the  angle  of  vertical 
divergence  is  not  at  disposal,  except  to  a  very  limited 
extent  in  some  cases,  but  is  determined  by  the  size  and 
character  of  the  flame.  With  the  electric  arc  and  a  large 
optical  apparatus  it  can  be  determined  in  considerable 
measure  how  the  light  shall  be  distributed  —  how  much 
shall  be  sent  to  the  distant  sea,  how  much  to  the  various 
distances  between  the  foot  of  the  tower  and  a  distance 
of  some  miles.  It  becomes  then  a  question  what  use  is  to 
be  made  of  this  facility.  The  experience  at  Macquarie 
and  at  Tino  is  emphatic,  that  it  is  in  every  way  advant 
ous  to  direct  much  the  greater  part  of  the  light  to 


horizon  with  a  very  small  divergence,  and  to  distribute  the  \^^ 
comparatively  small  remainder  over  the  nearer  sea  with  in- 
tensity increasing  with  the  distance. 

A  question  allied  to  the  last  is  this:  Whether  it  be  de- 
sirable to  provide  means  of  directing  the  strongest  light 
downwards  on  to  the  nearer  sea  in  time  of  fog?  The 
answer  must  depend  upon  the  circumstances  of  the  par- 
ticular locality.  Take  the  case  of  a  lighthouse  on  an 
isolated  rock,  the  purpose  of  which  is  primarily  to  be  a 
beacon  to  keep  ships  off  that  rock;  a  lighthouse  which 
would  not  exist  were  it  not  more  practical  or  cheaper  to 
build  and  maintain  the  lighthouse  rather  than  remove  the 
rock.  Here  surely  it  is  of  the  greatest  use  to  provide 
means  whereby,  if  the  light  cannot  penetrate  2  miles,  it 
shall  if  possible  be  visible  at  1  mile.  But  other  cases 
occur  in  which  the  lighthouse  has  to  cover  a  long  length 
of  coast,  and  has  almost  as  much  to  do  with  points  of  the 


242  DYNAMO   MACHINERY   AND  ALLIED  SUBJECTS. 

coast  10  miles  distant  as  with  the  point  upon  which  it  is 
placed,  cases  in  which  the  lighthouse  is  far  more  useful  in 
guiding  the  regular  traffic  passing  within  a  radius  of  20 
miles  or  more  than  in  preventing  vessels  running  ashore 
within  a  mile  of  the  tower.  Such  a  light  fails  of  its  pur- 
pose if  it  can  only  be  seen  at  a  distance  of  a  mile,  covering 
less  than  ^^  part  of  its  normal  area  of  illumination;  it 
becomes  comparatively  useless  unless  it  penetrates,  to 
something  like  its  normal  range,  and  its  efficiency  must  be 
measured  by  the  fewness  of  the  occasions  when  it  fails  to 
do  this.  It  is  a  grave  question  whether  it  be  prudent  in 
such  cases  to  place  upon  the  light  keeper  the  responsibility 
of  judging  when  the  light  should  be  dipped  on  to  the 
nearer  sea,  the  fact  being  that,  if  his  judgment  errs,  he 
may  actually  diminish  the  range  of  the  light,  and  cause 
unnecessarily  the  lighthouse  to  fail  of  fulfilling  its  most 
important  function.  It  is  easy  for  him  to  be  misled  if  the 
fog  is  local  and  does  not  extend  to  any  great  distance  from 
the  lighthouse.  Another  element  enters  into  the  con- 
sideration— the  height  above  the  sea.  If  the  focus  be  100 
feet  above  the  sea  level,  the  dip  of  the  sea  horizon  is  9' 
45",  and  a  ray  dipping  9'  45"  below  the  sea  horizon  will 
meet  the  sea  at  a  distance  of  3.1  nautical  miles  from  the 
tower.  Even  with  a  first  order  apparatus,  if  the  arc  be  a 
powerful  one,  it  is  very  difficult  to  render  the  light 
directed  to  the  horizon  from  an  elevation  of  100  feet  more 
powerful  than  that  directed  to  a  point  distant  4  miles  from 
the  tower.  Unavoidable  divergence  will  render  the  two 
intensities  practically  equal. 

Passing  to  questions  of  another  class,  what  are  the  rela- 
tive advantages  in  an  electric  lighthouse  of  continuous  and 


THE  ELECTRIC   LIGHTHOUSE  OF  TINO.          243 

alternating  currents?  Present  practice  tends  altogether 
in  favor  of  alternate  currents,  but  this  practice  largely 
results  from  unfavorable  experience  of  the  older  continu- 
ous current  machines.  These  machines  have  in  many 
respects  been  greatly  improved  in  the  last  two  or  three 
years.  The  continuous  current  presents  the  advantage  of 
greater  economy  of  power  in  producing  the  current,  less 
floor  space  required  by  the  machine,  and  a  smaller  prime 
cost.  The  alternate  current  magneto  machine,  on  the 
other  hand,  has  the  advantage  that  it  may  be  driven  with 
a  defectively  governed  prime  mover,  with  an  indifferent 
lamp,  and  may  suffer  neglect  with  impunity;  whereas  the 
more  compact  and  efficient  continuous  current  machine 
would  be  in  serious  peril  of  destruction.  Optical  apparatus 
can  be  constructed  suitable  to  make  the  most  of  either 
form  of  arc.  Hot  air  engines  have  found  favor  for  electric 
lighthouses,  because  in  many  cases  there  is  no  available 
supply  of  fresh  water.  The  engines  of  which  the  author 
has  experience  are  open  to  the  objection  that  they  take  a 
great  deal  of  room,  are  not  economical  of  fuel,  and  do  not 
govern  so  quickly  as  is  desirable;  the  wear  and  tear  also, 
when  they  are  worked  to  anything  like  their  full  power,  is 
very  serious.  A  gas  engine,  with  Dowson  or  other  gas 
made  on  the  spot,  could  be  used  with  greater  advantage, 

Antecedent  to  all  considerations  as  to  the  best  apparatus 
and  machinery  to  be  used  is  the  question  under  what  cir- 
cumstances, if  at  all,  should  electric  light  be  used  in  a 
lighthouse  ?  The  Trinity  House  experiments  at  the  South 
Foreland  showed  to  demonstration  that,  where  the  issue  to 
oe  decided  was  how  to  produce  a  light  which  should  be  ca- 
pable of  penetrating  the  furthest  in  all  weathers,  electric 


244  DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

light  could  do  that  which  could  be  done  in  no  other  way, 
and  that  it  was  the  cheapest  light  of  all  when  the  price  is 
estimated  per  unit  of  light.  But  the  conclusion  was  also 
reached  that  an  electric  light  must  inevitably  cost  a  large 
sum,  both  in  first  outlay  and  in  maintenance;  therefore 
that  electric  light  is  extravagant  unless  very  extraordinary 
power  is  a  necessity.  This  conclusion  is  doubtless  a  fair 
consequence  of  experience,  but  it  is  not  an  inherent  prop- 
erty of  electric  light.  Both  the  capital  outlay  and  the  cost 
of  maintenance  are  greatly  increased  by  the  practice  of  so 
arranging  the  machinery  as  to  provide,  at  all  times,  a  light 
of  very  great  power :  whence  it  follows  that  the  machinery 
must  be  placed  at  some  distance  from  the  lantern,  and  two 
men  must  always  be  on  duty;  one  man  in  the  lantern,  and 
another  with  the  machinery. 

The  essentials  for  a  cheap  electric  lighthouse  are,  that 
for  ordinary  states  of  the  atmosphere  there  shall  be  pro- 
vided a  plant  under  the  easy  control  of  the  light  keeper 
himself,  which  shall  be  precisely  adapted  to  produce  that 
amount  of  light  which  is  wanted  in  ordinary  states  of  the 
atmosphere;  but  for  thick  weather  there  shall  be  provided 
a  much  more  powerful  engine  and  dynamo,  available  also 
as  a  reserve  in  case  the  smaller  machinery  from  any  cause 
breaks  down.  The  occasional  machinery  may  be  more 
remote  from  the  lantern,  as  it  is  a  small  matter  to  require 
a  second  man  to  work  on  the  comparatively  rare  occasions 
when  the  maximum  power  is  needed.  A  small  gas  engine 
and  a  dynamo  machine  can  be  placed  without  any  crowd- 
ing in  the  room  immediately  below  the  lantern,  and  arrange- 
ments can  be  made  whereby  the  light  keeper,  whether  he 
is  in  the  lantern  or  in  the  engine  room,  can  ascertain  at  a 


THE   ELECTRIC    LIGHTHOUSE   OF  TINO.  245 

glance  whether  the  arc  is  in  its  proper  position,  with  an 
error  of  less  than  1  millimetre.  The  attendance  on  the 
lamp,  rotating  apparatus  of,  the  lens  (if  a  revolving  light), 
engine  and  dynamo,  would  be  easy  when  the  whole  is 
brought  together  so  as  to  be  under  observation  at  once;  in 
fact  the  gas  engine,  dynamo,  and  lamp  constitute  together 
a  gas  burner  which,  though  consisting  of  many  parts,  is 
automatic  throughout,  and  requires  nothing  but  the  con- 
stant presence  of  a  custodian,  exactly  as  the  gas  lamp  in  a 
lighthouse  requires  a  custodian  as  a  guarantee  against  fail- 
ure. The  same  end,  viz.,  the  concentration  of  the  whole 
mechanical  and  electrical  apparatus  under  one  pair  of  eyes, 
could  be  attained,  of  course,  in  other  ways.  Accumulators 
could  be  used,  or  a  petroleum  engine. 

In  order  to  give  definiteness  and  afford  facilities  for 
criticism,  the  better  course  will  be  to  describe  a  suitable 
machinery;  state  what  it  will  do,  what  attendance  it  will 
require,  and  what  it  will  cost.  The  author  proposes,  then, 
for  an  electric  lighthouse  where  small  outlay  is  essential, 
the  following:  A  Dowson  gas  producing  apparatus  and  gas 
holder,  the  generator  and  superheater  being  in  duplicate, 
each  capable  of  making  1,200  feet  of  gas  per  hour,  the  gas 
holder  having  a  capacity  of  3,000  cubic  feet. 

An  8  h.  p.  nominal  Otto  gas  engine  and  series  wound 
dynamo  machine,  placed  in  a  room  near  the  base  of  the 
tower,  and  copper  conductors  to  the  lantern,  the  dynamo 
having  magnet  coils,  divided 'into  sections  so  as  to  supply  a 
small  current  when  required. 

A  1  h.  p.  nominal  Otto  gas  engine  and  dynamo  machine, 
placed  in  the  room  immediately  beneath  the  lantern  floor, 
with  gas  pipe  from  the  gas  holder;  three  electric  lamps,  to 


246   DYNAMO   MACHINERY   AND   ALLIED   SUBJECTS. 

receive  either  carbons  25  millimetres  in  diameter  or  any 
lesser  size,  with  complete  adjustments  for  accurate  focus- 
sing; one  paraffin  lamp  as  a  substitute;  an  optical  appara- 
tus of  the  second  order  of  70  centimetre  focal  distance. 
The  cost  of  this  apparatus  would  depend  upon  the  charac- 
ter of  the  light  it  was  intended  to  exhibit.  To  fix  ideas, 
let  it  be  assumed  that  the  light  is  to  be  a  half  minute 
revolving  light,  showing  all  round  the  lighthouse.  There 
could  then  be  supplied  a  sixteen  sided  apparatus  with  ped- 
estal and  revolving  machinery.  Provision  would  be  made 
in  the  optical  apparatus  for  giving  the  horizontal  and  ver- 
tical divergence  desired  by  the  same  methods  successfully 
used  in  the  lighthouses  of  Macquarie  and  of  Tino. 

Two  focussing  prisms  would  be  fixed  to  form  magnified 
images  of  the  arc,  on  pieces  of  obscured  glass  let  into  the 
pedestal  floor,  so  that  the  keeper,  whether  in  the  lantern  or 
in  the  engine  room,  could  see  at  a  glance  the  state  of  the 
arc,  and  observe  whether  it  is  of  proper  length  with  the 
carbons  in  line,  whether  it  is  exactly  at  the  right  height 
and  in  the  centre  of  the  apparatus.  An  error  of  I  millim- 
etre would  bu  glaringly  apparent,  and  call  for  immediate 
adjustment,  although  its  effect  would  be  only  a  displace- 
ment of  the  beam  5'  of  angle. 

The  lantern  would  be  10  feet  diameter,  with  bent  plate 
glass. 

The  cost  of  the  whole  above  described  would  be  materi- 
ally less  than  the  cost  of  a  first  order  light  and  lantern 
with  oil  lamp  and  large  burners. 

Now  what  result  would  be  obtained  ?  In  fine  weather 
the  small  engine  would  be  used.  Its  effective  power  on 
the  brake  is  fully  1£  h.  p.;  from  this  1J  h.  p.  the  dynamo 


THE  ELECTRIC   LIGHTHOUSE  OF  TINO.          247 

machine  produces  considerably  over  800  watts,  say  800 
watts  in  the  arc  itself,  or  20  amperes  through  a  fairly  long 
arc  of  40  volts.  Of  course  the  value  of  this  in  candles  de- 
pends upon  the  color  in  which  it  is  measured,  and  the 
direction  in  relation  to  the  axis  of  the  carbons.  In  red 
light  the  mean  over  the  sphere  would  certainly  exceed 
1,200  candles.  In  clear  weather  or  in  slight  haze  or  rain, 
the  beam  of  this  light  through  the  lenses  would  be  much 
more  powerful  at  the  horizon  and  on  the  more  distant  sea 
than  any  single  focus  light  with  oil  or  gas  as  the  illumi- 
nant,  and  would  at  least  be  fairly  comparable  with  any- 
thing yet  exhibited  with  oil  or  gas  whether  triform  or 
quadriform.  But  on  the  nearer  sea  the  illumination 
would  be  reduced,  so  that  no  annoyance  would  be  caused 
by  dazzling  flashes.  In  thick  weather  or  indeed  in  any 
weather  when  there  was  a  doubt  as  to  the  visibility  at  the 
horizon  of  the  lower  power,  the  large  engine  would  be 
used  under  the  superintendence  of  the  second  keeper. 
This  engine  will  give  10  h.  p.  on  the  brake,  and  there  is  no 
difficulty  in  obtaining  85  per  cent,  of  this  as  useful 
electrical  energy  outside  the  machine,  that  is,  6,340  watts. 
From  this  deduct  10  per  cent,  for  the  leads  and  the  lamp 
and  for  steadying  the  arc,  leaving  5,710  watts  in  the  arc 
itself,  or  114  amperes,  with  a  difference  of  potential  of  CO 
volts.  Having  regard  to  the  fact  that  the  optical  appa- 
ratus here  proposed  acts  upon  a  larger  portion  of  the 
sphere  than  that  used  in  the  South  Foreland  experiments, 
that  the  vertical  divergence  is  less,  and  that  the  potential 
difference  is  greater  and  the  current  continuous,  although 
less  in  quantity,  it  may  safely  be  assumed  that  the  power  of 
the  resulting  beam  would  not  be  inferior.  It  hence  follows, 


248  DYNAMO  MACHINERY   AND   ALLIED   SUBJECTS. 

from  the  South  Foreland  experiments,  that  in  any  fog  the 
flashes  would  penetrate  farther  than  those  of  any  existing 
gas  or  oil  light.  The  increased  size  of  crater,  compared 
with  that  produced  by  the  current  of  20  amperes,  will  give 
increased  vertical  divergence,  and  so  cause  the  maximum 
illumination  to  be  attained  at  a  less  distance  from  the 
lighthouse.  The  attendance  of  two  men  would  suffice  for 
all  the  duties  of  the  lighthouse,  because  under  ordinary 
circumstances  one  man  only  need  be  on  duty  excepting  for 
two  to  three  hours  while  gas  is  being  made.  The  con- 
sumption of  coal  would  he  4  Ibs.  per  hour  of  lighting,  of 
water  about  £  gallon,  of  carbons  about  4  inches.  The 
whole  cost  of  maintaining  the  light  would  differ  little  from 
that  of  an  ordinary  oil  light  of  the  first  order. 

Though  it  be  the  fact,  that  it  is  possible  to  exhibit  an 
electric  light  at  moderate  cost,  it  does  not  follow  that  it  is 
suitable  for  all  ocean  lights.  There  is  no  room  in  a  rock 
lighthouse  tower  for  a  gas  plant,  and  few  would  at  present 
be  prepared  to  recommend  a  petroleum  engine  burning  oil 
of  a  low  flashing  point.  The  light  keeper  again  must 
understand  a  gas  producer,  a  gas  engine,  a  dynamo,  and 
an  arc  lamp,  instead  of  only  a  paraffin  lamp  and  burner, 
and  arrangements  must  exist  for  repairing  the  more  ex- 
tensive machinery.  Such  considerations  will  justly  weigh 
against  the  use  of  the  electric  light  in  remote  stations  and 
in  countries  where  the  labor  available  is  not  capable  of 
much  training. 

It  may  possibly  be  said  that  in  this  paper  no  definite 
conclusions  are  reached  as  to  whether  electricity  or  some 
other  agent  is  the  best  source  of  light  in  lighthouses 


THE   ELECTRIC   LIGHTHOUSE  OF  TINO.  249 

generally,  nor  yet,  if  electricity  be  adopted,  what  is  the 
best  way  of  producing  the  light  and  optically  dealing  with 
it.  The  answer  is  that  it  is  impossible  on  many  points  to 
arrive  at  general  conclusions.  Each  case  must  be  judged 
according  to  its  special  circumstances. 


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